The effect of receiving tritiated water (HTO) throughout gestation on the developing primate was assessed by administering HTO to adult female squirrel monkeys (Saimiri sciureus) as the only source of drinking water beginning with the day of insemination and continuing throughout pregnancy. For the control (tap water) and six experimental groups, the mean urinary tritium concentrations in females delivering full-term progeny were <0.004, 0.05, 0.16, 0.33, 0.75, 1.61, and 3.09 microcuries/ml. Positive bioassays for pregnancy were observed in about half of 277 inseminated females. Among pregnant females, the full-term delivery rate was 36%, the abortion rate was 7%, and the resorption rate was 58% with no discernible effect of HTO administration on any of these parameters. The 46 full-term progeny were evaluated within 2 days of birth. No effects of HTO administration were observed in terms of gestation period (median 153 days, range 141-158 days), sex distribution, body weight, body dimensions, selected organ weights, histology (except gonads), or hematologic pattern. The number of primary oocytes in female progeny decreased markedly within increasing concentrations of tritium. Specific quantification of this effect and evaluation of the neonate testes is in progress.