Female rats have been exposed (24 hours for 15 days) to N2O 60% in normo-oxic blend of N2 and O2. Vaginal smears have been observed daily. At the end of the experiment the ovaries have been removed and the following morphological parameters have been statistically evaluated: number of evolutive follicles and corpora lutea, amount of interstitial tissue. Our results seem to account for a remarkable toxic effect of N2O on the rat gonad. Such effect is testified by alterations of the oestrous cycle and, at the ovary, a decrease in the number of evolutive follicles and functioning corpora lutea and by a sharp increase of atretic phenomena.