Several unique in vitro test systems which measure biological activities of luteinizing hormones have been developed. Luteinizing hormone (LH) or human chorionic gonadotropin (HCG) markedly stimulates the early steps of purine biosynthesis in supernatant prepared from homogenates of corpora lutea from the pig, cow and human ovary. This stimulatory effect of LH and HCG is due to activation of corpus luteum glucose-6-phosphate dehydrogenase leading to an increased generation of 5'-phosphoribosylpyrophosphate (PP-Rib-P) by the pentose phosphate pathway. PP-Rib-P is ratelimiting to 5'-phosphoribosylpyrophosphate amidotransferase, the first enzyme of de novo purine biosynthesis. PP- Rib-P is also rate-limiting to the biosynthesis of pyrimidine nucleotides. The alpha and beta subunits of LH and HCG inhibit the stimulatory effect of the intact luteinizing hormone. The competitive interaction appears to be at the level of glucose-6-phosphate dehydrogenase in the corpus luteum. Preliminary experiments have indicated that LH and HCG stimulate RNA synthesis in isolated nuclei obtained from corpora lutea. All types of RNA appear to be increased by the trophic hormones in the presence of added glucose-6-P. The basic information obtained from the interaction of luteinizing hormones, the alpha and beta subunits, smaller fragments from the beta subunits and chemically modified hormones will be used to determine the active center for luteinizing hormones. It is hoped that specific inhibitors could be designed that would block the corpus luteum receptors for HCG to serve as contraceptive compounds in early pregnancy.