While the necessity for progesterone administration throughout pregnancy in the ovariectomized rabbit is not questioned, the roles of 20oc-dihydroprogesterone and oestradiol are stilI in doubt.20oc-dihydroprogesterone was shown to be a weak inducer of implantation with less than one-tenth the potency of progesterone. The significance of its high level of production on the day after mating remains obscure.In combination with the earlier results of Kwun and Emmens (1974), further work with oestradiol suggests that at no stage is it clearly needed for successful maintenance of pregnancy. However, in low doses (0·125-O·2Ilg/day prior to implantation, 0·2 rising to 1·6Ilg, or remaining at 0·2Ilg/day thereafter) it produced slight but sometimes significant improvements in implantation and foetal development percentages.Birth processes were abnormal if progesterone injections were continued beyond day 29. Foetuses were most frequently retained in utero or born dead after a somewhat prolonged pregnancy. The cessation of injections on day 29, whether or not a low dosage of 0·2 Ilg of oestradiol per day were continued, resulted in 94-98 % normal parturition, but the percentage of live births was stilI significantly below that of controls unless oestradiol was given.