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SCIENTIFIC ABSTRACT We propose to probe the interstellar medium in the z=1.1 radio galaxy 3C368 (autumn launch) or 3C280 (spring launch) with SWS, PHOT and LWS, in order to gain insight into the relative importance of star formation and active nucleus in these distant objects. Strong indications for active star formation have been found in photometry, imaging and imaging spectroscopy in distant radio galaxies. These include (1) the alignment effect between the radio axis and optical/near-IR continuum light (2) extended emission line regions (in e.g. Lyman alpha, [O II], [O III]) aligned with the optical/radio continuum axis (3) spectral energy distributions (SEDs) which show a strong blue component (4) mm-wave continuum detections far exceeding the expected synchrotron flux and thus attributed to thermal dust emission (5) the detection of large quantities of molecular gas in a number of radio galaxies with z between 1 and 3 (including one of our candidate sources). The properties (1)-(3) are interpreted in terms of intense star formation induced by the radio jet, or scattering of the nuclear light. Both interpretations have problems, and it is likely that a combination of both is at work. The relative importance of star formation and active nucleus for the observed properties is one of the most outstanding question in the study of these objects. Properties (4)-(5) above show that the raw material for forming stars is abundantly present, and bring to mind the probable primeval galaxy IRAS F10214+4724. They also directly show that these objects are very dusty and their nuclear regions can therefore only be probed at mid-IR and longer wavelengths. The brightest and most important lines fall in wavelength regions that are inaccessible from the ground, and are too faint to be observed from the KAO. They can thus ONLY be observed with ISO. Therefore, in order to study the properties and excitation of the interstellar medium (ISM) in these objects we will (1) use PHOT to measure intensity and spectral index of the dust emission on the (rest frame) Wien side of the dust emission spectrum, yielding dust temperature and luminosity, (2) use SWS and LWS to measure 5 bright and diagnostic fine-structure lines, covering a range of excitation potentials. Line strengths (or useful limits) and ratios will allow us to determine such parameters as slope and cutoff of the ionizing continuum, and ionization parameter (unhindered by obscuring dust), with straightforward implications for the relative importance of star formation and active nucleus. Since high z radio galaxies are currently our only way to probe galaxies at relatively early cosmological epochs, the result will have direct implications for our understanding of the early evolution of galaxies. The ISO observations will be complemented with ground-based measurements, e.g., of the submm continuum. OBSERVATION SUMMARY Line measurements: selected lines are listed below under Scientific Justification. With this choice we (1) cover a sufficient range of excitation conditions (2) optimize for the brightest lines (3) avoid uncertain abundance corrections by using ratios of lines from the same species, and (4) probe various phases of the ISM including photodissociation regions ([O I]), HII regions ([Ne II], [O III]), AGN narrow line region ([O III], [Ne III]) and coronal line region ([O IV]). As an illustration of the diagnostic power of this selection, we note that for a starburst model [Ne II] / [Ne III] >> 1, whereas in an AGN model [Ne II] / [Ne III] << 1. The detection of [O IV] directly implies the hard UV continuum of an AGN, where a coronal line region strongly radiates in this line. Line strength predictions are based on observations of M82 and NGC1068 for [Ne II], [O I] and [O III], scaled by CO luminosity and placed at a redshift of 1.1. For [Ne III] and [O IV] model predictions are used, scaled in a similar way. This yields e.g. an estimated 1e-17 W/m2 for [Ne II] which is within easy reach with the SWS. Our calculations indicate that the neon lines are expected to be detected at high significance in about 15 minutes of integration time (per line). For the [O I] line we expect a S/N of 4 after 30 minutes, while (in case of an AGN) [O IV] will give S/N=3 in 30 minutes. The same approach has been used to predict the continuum strengths. During the SWS observations, simultaneous measurements will be carried out of the H2 v=1-0 S(1) line (redshifted to 4.5 micron) and the [Si IX] 3.935 micron line (which is bright in Seyfert nuclei). These lines do not form part of the main programme and are merely included in order to use the freely available time in the other SWS band.