Follicular fluid (FF) levels of immuno reactive beta-Endorphin (i.r. beta-EP), i.r. gamma-Endorphin (i.r. gamma-EP), i.r. alpha-melanocyte-stimulating hormone (i.r. alpha-MSH), androgens and estrogens were measured in 76 preovulatory follicles obtained, after gonadotropin stimulation from 19 women undergoing in vitro fertilization (IVF). The aim of the study was to investigate the relationships existing between peptide contents of FF and both oocyte-cumulus-corona-complex (OCCC) maturity and the success of IVF. Peptides and steroids were measured by RIA after FF extraction with liquid chromatography and ethyl-ether, respectively. Out of the total of 76 oocytes, 52 were fertilized in vitro and 35 of them underwent normal cleavage and were transferred. Among the three peptides, only i.r. beta-EP levels were higher in FF from follicles which contained oocytes that were subsequently fertilized (127.6 +/- 16.2 pmol/L mean +/- SE) than in FF from follicles which contained oocytes that did not subsequently fertilized (62.9 +/- 8.4, p less than 0.04). Independent of subsequent fertilization, i.r. alpha-MSH values in FF were 5 times higher than those of i.r. beta-EP and i.r. gamma-EP. In the presence of a morphologically mature oocyte, FF i.r. gamma-EP levels (165.2 +/- 45.3 pmol/L) were higher than in FF from follicles yielding immature (63.6 +/- 13.5, p less than 0.01) or luteinized (32.7 +/- 9.2, p less than 0.01) oocytes. Steroid levels in FF did not change in relation to oocyte maturity or subsequent oocyte fertilization.(ABSTRACT TRUNCATED AT 250 WORDS)