We measured the progesterone, estradiol (E2), serotonin (5-HT), noradrenaline (NA) and dopamine (DA) contents of follicular fluid (FF) samples obtained from 35 patients undergoing in vitro fertilization and embryo transfer. Progesterone and E2 were determined by radioimmunoassay, and monoamines were measured by a spectrofluorimetric method. Significantly higher progesterone and NA levels were found (p < 0.01) in FF from cycles in which the oocyte cleaved and resulted in pregnancy compared to FF containing uncleaved oocytes. NA and DA contents were significantly higher in FF from cycles resulting in pregnancy than in FF containing cleaved oocytes not resulting in pregnancy (p < 0.01). There were significantly lower NA and 5-HT levels in FF containing uncleaved oocytes compared to the latter group (p < 0.05). Significant positive correlations were found between progesterone and 5-HT (r = 0.67) and between NA and DA (r = 0.93) in the pregnant group; between progesterone and E2 (r = 0.84) in FF containing uncleaved oocytes; between progesterone and E2 (r = 0.71) and between NA and DA (r = 0.62) in FF from cycles in which the oocyte cleaved but did not result in pregnancy. The results suggest that the follicular hormonal changes associated with oocyte maturation may be locally modulated by monoamines.