Studies were conducted to determine the teratogenic potential of theobromine (TBR) and cocoa powder (CP) in rabbits. TBR was given either by gavage at dose levels of 0, 25, 75, 125 or 200 mg/kg body weight/day or administered in the diet at 0, 0.0625, 0.125 or 0.1875% (approximately 0, 21, 41 or 63 mg/kg/day, respectively). CP was given at 2.5, 5.0 or 7.5% of the diet (approximately 25, 50 or 75 mg methylxanthines/kg body weight/day). The duration of exposure was from days 6 to 29 of gestation. Significant maternal mortality (40%) and reduced food consumption were observed at 200 mg TBR/kg/day. Mean foetal weights were similar to those of the control group at 25 or 75 mg TBR/kg/day, but decreases in foetal body weight and increases in various malformations and developmental variations were observed in groups given 125 or 200 mg/kg/day. Insufficient litters were available for examination in the 200-mg/kg/day dose group because of maternal toxicity/lethality (repetitive exposure by gavage to 200 mg TBR/kg approached the maternal LD50). In the dietary CP studies, three does died and three aborted, but these deaths and abortions were not treatment related. No maternal deaths occurred during dietary TBR exposure. Maternal weight gain and food consumption, and the mean number of corpora lutea were unaffected by either dietary CP or TBR. Neither foetotoxicity nor teratogenicity was associated with dietary ingestion of CP or TBR. The foetuses exposed to 0.125% or 0.1875% TBR had a significantly higher incidence of incompletely ossified or absent sternebrae, whereas exposure to 0.1875% or 7.5% CP resulted in corresponding effects on metacarpal bones, indicating a delay in osteogenesis. The predominant compound found in serum after TBR ingestion was unchanged TBR, and there was no evidence of bioaccumulation of TBR in serum during gestation. The highest levels of CP or TBR used in these studies was 38 times greater than the maximum consumption level reported for humans in marketing surveys, and corresponds to a consumption of greater than 7.5 lb milk chocolate/day.