[en] One possible energy source for infrared-emitting galaxies is many newly formed stars. This idea is tested by observing far infrared emission lines of interstellar oxygen and nitrogen gas in the galaxy M82 and the colliding galaxy system Arp 299. Observed fluxes in 4 lines in M82 roughly match those calculated from existing models of M82 that predict active massive-star formation. In Arp 299, an upper limit is measured on the flux in one line of doubly-ionized oxygen (⁺⁺), also consistent with fluxes predicted. These arguments support the picture that M82 and Arp 299 are powered by many newly formed high-mass stars. The other results are: (1) All 3 oxygen lines in M82 show a strong narrow feature in addition to the expected Doppler-broadened emission peak. This implies an unusual source of line radiation, which does not emit the nitrogen line strongly, in M82. The nature of this source is discussed. (2) The abundance ratio of twice-ionized nitrogen (N⁺⁺) to O⁺⁺ in the broad source in M82 is higher than in the narrow source and comparable to galactic values. Average electron densities in M82 were found 10 to 100 times lower than in typically-observed HII regions in the Galaxy. Lower densities may result from winds from frequent supernovae in M82