• bioactive luteinizing hormone;
  • estrous cycle;
  • seasonal changes;
  • releasing hormone challenge


Studies on the reproductive mechanisms of the squirrel monkey have been hampered by inadequate measurements of luteinizing hormone (LH). The mouse interstitial cell bioassay, which measures testosterone production as the endpoint, was validated for use in the squirrel monkey by parallel responses of serum to LH standards and by in vivo responses to an LH-releasing hormone (LHRH) analogue. The LH surge profile, as determined by daily blood sampling, was found to be of 1–2 days duration and comparable in amplitude to those of other primates. A 9.7-day ovulatory cycle length was also calculated and was similar to previous estimates based on other hormonal and behavioral methods. A 150-fold decrease in basal LH was found in the nonbreeding season, as was a decreased LHRH response. This assay makes possible future studies on hypothalamic-ovarian mechanisms in this species.