Of a total of 343 pregnant rats treated with the prostaglandin F2alpha analogue ICI 81008, 137 aborted, while 83 had reduced and 123 intact litter. These biological variations depended primarily on the gestational timing of treatment and on the dose and route of administration of this synthetic PG. In comparison with the 107 controls, all experimental rats which aborted had a drastic reduction in plasma progesterone levels which was highly significant (P is less than 0.001) until day 18. In contrast, those animals which escaped suboptimal ICI 81001 treatment with a partly resorbed litter, only had a moderate reduction in progesterone which was statistically significant (P is less than 0.01) until day 16 when levels of this steroid normally begin to decrease. Ineffective treatment did not affect progesterone levels and intact pregnancy. In contrast to progesterone, there was no correlation between plasma estradiol-17beta levels and the consequences of ICI 81008 treatment.