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SCIENTIFIC ABSTRACT It is proposed to use the ISO Long Wavelength Spectrometer (LWS), the Short Wavelength Spectrometer (SWS) and the Camera (CAM) to investigate the infrared spectra of compact HII regions. The selected sample of 60 sources covers in a nearly homogeneous way the galactic plane from the center out to 25 kpc. Together with the study of the physical conditions pertaining in these regions of recent massive star formation, this will enable us to investigate in great detail the problem of the present-day abundance distribution in the Galaxy. The study of chemical abundances and their variation as a function of the distance to the galactic center is of fundamental importance for our understanding of galactic evolution. The abundances of heavy elements in the interstellar medium provides a direct estimate of the enrichment due to nucleosynthesis in successive generations of stars. The variation of one element relative to another yields information on the formation paths of these elements. A detailed knowledge of the abundance variation within a galaxy is a prerequisite to a wide variety of studies such as the large scale distribution of the infrared radiation, gamma rays and cosmic rays. The infrared fine-structure lines are better suited than the optical lines (as usually used) for abundance analysis in the Galaxy for two reasons. First, the infrared lines are comparatively insensitive to the electron temperature so that the correction for electron temperature, crucial in the optical studies, is not required. Second, infrared lines are less attenuated by interstellar extinction than their optical counterparts and provide a probe of the elemental abundances through the galactic disk (although substantial extinction corrections are still necessary), in particular in the inner regions, at galactocentric distances less than 6 kpc where no optical measurements are possible. OBSERVATION SUMMARY The compact HII regions have been selected according to the following criteria: (1) The sources should be compact so as to be included in the field of view of the Short Wavelength Spectrometer (20 arsec x 20 arcsec) and isolated so as to avoid confusion problems with the larger field of view of the Long Wavelength Spectrometer (1.65 arcmin). (2) The sample should be uniformly distributed in galactocentric distance. Compact HII regions near the galactic center (only observable in the infrared and radio wavelengths) and sources associated with the molecular ring (at 5 kpc, where HII regions are expected to be numerous and share similar abundance properties) should be well represented in the sample. (3) The selection is mainly based on both the sample of about 200 compact HII regions extracted from the IRAS-LRS database (see Jourdain de Muizon et al., 1989, A&A Suppl. 83, 337) and the study of star-forming regions in the outer galaxy by Wouterloot and Brand (1990, A&A Suppl. 80, 149) for the sources at distances greater than 10 kpc from the galactic centre. (4) The fluxes in the four IRAS bands (12, 25, 60 and 100 um) should be strong so as to minimize the integration times: the infrared luminosity of compact HII regions being nearly equal to the luminosity output of the exciting star, strong infrared fluxes should indicate strong fluxes in the fine-structure lines. However we relaxed this criteria for regions far away from the galactic centre (i.e. at distances greater than 15 kpc). The final sample contains 60 sources which are presented in Table 1. Due to time constraints a sub-sample of 30 sources was extracted for observations with SWS and CAM.