Timed-pregnant CD rats and CD-1 mice were dosed by gavage with oxytetracycline hydrochloride (OXT) in corn oil on gestational days (gd) 6-15 (0, 1200, 1350, or 1500 mg/kg/day for rats; 0, 1325, 1670, or 2100 mg/kg/day for mice). Deaths among treated females occurred in a dose-related manner in all OXT dose groups (2-7%, mice; 5-24%, rats), but no maternal deaths occurred in the vehicle control groups. Significant dose-related decreases in maternal weight gain during treatment, as well as for corrected gestational weight gain (i.e., maternal gestational weight gain minus gravid uterine weight), were observed at all doses in rats but not in mice. Gravid uterine weight was reduced in a dose-related manner only in mice, with the high-dose group significantly reduced compared to the control group. At termination (gd 20, rats; gd 17, mice), the status of uterine implantation sites was recorded and live fetuses were weighed. Fetuses were examined for external, visceral, and skeletal abnormalities. There were no significant effects of OXT in either species on the incidence of postimplantation loss (resorptions plus dead fetuses) or malformations. In both species, there was a significant trend toward reduced fetal body weight, and each group of rats receiving OXT was significantly reduced compared to the control group. Administration of OXT during organogenesis at doses exceeding the therapeutic range for humans produced maternal and fetal toxicity, but did not produce any treatment-related increase in malformations.