The fetotoxicity of cyclosporin A (CsA) was examined in multiparous Sprague-Dawley rats given the drug (25 mg/kg/day) during different phases of gestation, the effects on the outcome of pregnancy being ascertained on day 19. CsA given from days 1 to 7 caused a small but significant reduction in litter size, with no significant increase in the number of resorptions. When the drug was administered from day 8 to 14 there was no significant change in litter size, but a very striking increase in the incidence of resorptions. This fetotoxic effect was also evident but less marked when the drug was witheld until day 15. Reduction in fetal weight was only present in the group given CsA from days 8 to 14. In surviving fetuses the presence of focal decidual necrosis was more frequent in mothers receiving CsA, suggesting a possible mechanism whereby CsA may mediate its fetotoxic effects.