The effects of exposure to nitrous oxide on reproductive indices, fetal development, and male fertility were examined in Swiss/ICR mice. In experiment I, female mice were exposed for 4 hours per day on days 6-15 of pregnancy, to 0.5% (5,000 ppm), 5.0% (50,000 ppm), or 50% (500,000 ppm) nitrous oxide. Control mice were untreated, exposed to compressed air, or treated with retinoic acid on day 8 of gestation. In experiment II, male mice were treated, as above, for 9 weeks and then mated nightly for 7 nights to untreated, virgin females. In experiment I, 1,761 fetuses from 154 dams were examined and found to be without evidence of adverse nitrous oxide treatment effects. In experiment II there were no differences among the groups in the ability of males to impregnate females or in litter size, fetal wastage, or fetal size. When we compare nitrous oxide with other inhalation anesthetics we have studied employing a similar protocol, we find the order of reproductive toxicity to be: halothane greater than enflurane greater than methoxyflurane greater than nitrous oxide. None of the agents were toxic, however, at the trace concentrations usually found in operating rooms.