The effect of luteinizing hormone antiserum (LH A/S) on sterol and steroid levels in luteal and non-luteal ovarian compartments, and on serum steroid levels of pregnant hamsters, was studied. Injection (i.p.) of 0.1 ml of LH A/S on day 7 caused termination of pregnancy and morphological regression of the corpus luteum (CL) within 18-20 h of treatment, whereas LH-free non-immune serum had no effect. Within 3 h of administration of LH A/S the luteal progesterone levels fell from a control value of 36.0 .+-. 2.4 to 15.6 .+-. 3.4 ng/mg, and estrogen from a control value of 20.8 .+-. 3.4 to 12.2 .+-. 1.5 pg/mg. By 12 h progesterone had dropped to 7.3 .+-. 0.3 ng/mg whereas estrogen was undetectable by radioimmunoassay. Serum progesterone fell in 3 h from 5.53 .+-. 0.39 to 3.15 .+-. 0.26 ng/ml whereas estrogen fell from 172 .+-. 26 to 123 .+-. 9 pg/ml. The progesterone content of non-luteal ovarian tissue dropped in 3 h from 1.06 .+-. 0.01 to 0.47 .+-. 0.06 ng/mg and estrogen from 115.0 .+-. 1.5 to 11.0 .+-. 0.65 pg/mg. Both progesterone and estrogen were undetectable at 24 h. In the CL, free cholesterol concentration did not change significantly after A/S treatment, while esterified cholesterol increased from 2.82 .+-. 0.14 to 4.14 .+-. 0.25 .mu.g/mg within 6 h and 5.52 .+-. 0.17 .mu.g/mg by 12 h. LH may play a critical role in the maintenance of CL of early pregnancy in the hamster and deprivation of LH, even for periods as short as 3 h, significantly influences steroid production. The accumulation of cholesterol esters following LH-deprivation suggests that loss of cholesteryl esterase activity could be 1 of the events leading to luteolysis.