Male frogs (Rana pipiens) were injected intraperitoneally with triethylenemelamine (TEM). The injected males were held for seven days to permit TEM interaction with sperm. The TEM-treated males were then spermiated with human chorionic gonadotropin (HCG) and the ova of normal females were inseminated with the sperm. Direct observation was made of in vitro fertilized ova, cleavage, blastulation, and subsequent embryonic development. No differences were detected between the controls and treated animals from any experimental group for fertilization, cleavage, and blastulation. Dose-related morphologic abnormalities first became apparent at the time of gastrulation; the capability of hatched embryos to swim was affected by TEM. Metaphase chromosomes were studied from randomly selected embryos, and the prevalence of chromosomal structural abnormalities increased with dose over a range of 13--1,300 microgram TEM per kilogram frog.