One objective of this study is to prepare compounds, probably all steroidal, which are effective against estro-dependent processes associated with reproduction. These compounds would act specifically on the estrogen synthetase enzyme system and inhibit estrogen biosynthesis. Depletion of estrogen at certain periods should block ovulation in many lower animals and in humans and perhaps interfere with implantation as well. The second objective is to evaluate further an in vitro estrogen synthetase inhibitor which we have developed which acts as an antifertility agent in the rat. It depresses ovarian estrogen secretion in vivo and is effective in preventing implantation in the rat when administered both pre-coitally and post-coitally. We plan to determine in the rat if there are dose-dependent relationships among the amount of inhibitor administered, the degree of estrogen suppression and reproduction processes. Effects on LH, FSH and testosterone also will be determined. Gas Chromatography-Mass Spectrometry probably will be used to determine inhibitor plasma levels. Additional studies to establish the mode of action of the inhibitor will be carried out in hamsters, rabbits and possibly primates.