Disulfiiam, in 1 mg and 10 mg oral doses, was given to inbred C3H mice prior to and for the duration of pregnancy. The effects on the fetus have been assessed at 18 days gestation by means of fetal and placental weights, number of resorptions, skeletal preparations and Wilson's sections. Disulfiam dissolved in diethyl ether was administered to 8and 9-day embryos in vitro in concentrations of 0.1, 10 and 100 pg ml-' of culture medium, and the effects of ether alone and ether plus disulfiram assessed by evaluation of morphological development over a 28-h period, and inhibition of DNA synthesis using tritiated thymidine labelling over a 4-h period. Disulfiiam (1 mg) in vivo caused no adverse effects on the fetus, but disulfiiam (10 mg) was toxic, in tnat it caused a significant increase in early resorptions. Disulfuam in vitro, in the 10 and 100 pg ml-' concentrations, proved to be very toxic to the embryos, affecting both morphological development and DNA synthesis in 9-day embryos and morphological development in 8-day embryos. DNA synthesis was only inhibited at the 100 pg ml-' concentration in 8-day embryos. The 0.1 pg ml-' concentration of disulfiram caused abnormal central nervous system development in 8-day embryos, but was otherwise non-toxic to 8-and 9-day embryos. Apart from a reduction in somite counts, ether in concentrations of 0.285 mg ml-' and 2.85 mg ml-' caused no adverse effects on morphological development in 8-or 9day embryos. DNA synthesis was inhibited by ether in a concentration of 2.85 mg mi-' in 9-day embryos.