Caffeine exposure in utero in rats is known to result in intrauterine growth retardation and lowered birth weight as well as changes to behaviour and brain biochemistry. We have investigated whether caffeine's embryotoxicity is a result of the events associated with increased hsp synthesis, i.e., disruption to normal protein synthesis. Caffeine (30 mg/kg) was administered orally to pregnant rats as single or repeated doses. Embryos were removed 3 h after dosing on gestation day (GD) 9, 10, 11 and 12 and total embryonic protein and RNA analysed. There was no change in the mRNA or protein levels of hsp 88, 71/73, and 25 after acute or chronic treatment. To separate the direct effect of caffeine from those mediated through the mother, whole rat embryo culture was used. Caffeine (50 micrograms/ml) for 90 min did not increase hsp 88, 73 or 25 mRNA levels in 9.5, 10.5 and 11.5 GD cultured embryos. We conclude that in vivo or in vitro treatment of 9-12 GD rat embryos with moderate to high doses of caffeine does not increase the synthesis of the major mammalian hsps. Hence, hsp induction is unlikely to play a role in the embryotoxic actions of caffeine.