The purpose of the present study was to investigate the possible role of intrafollicular amino acids in the human ovarian follicle. Follicular fluid (FF) and oocytes were obtained from 12 women for in vitro fertilization. Follicular development was induced with clomiphene citrate and human menopausal gonadotropin. Thirty-six FF samples, free of visible blood contamination and containing mature oocytes were used to measure free amino acids, steroids and gonadotropins. The FF were divided into three groups: (1) follicles yielding oocytes that were unfertilized (n = 12); (B) follicles containing oocytes that were fertilized and cleaved to less than four cells (n = 12); (C) follicles yielding oocytes that were fertilized and cleaved to four or more cells (n = 12). The concentrations of histidine, phenylalanine and asparagine in FF were significantly greater than those in plasma at the time of follicle aspiration. However, amino acid concentrations in FF did not differ significantly between the three groups studied. No significant relationships were found between intrafollicular levels of amino acids and those of LH, FSH, estradiol, progesterone and testosterone. These results suggest that intrafollicular amino acids are not involved in the regulation of human preovulatory follicle.