To demonstrate the presence of somatostatin in human pre-ovulatory follicular fluid, and to assess the role of this peptide in follicular maturation, a total of 66 follicular fluid samples were obtained from 26 patients at the time of oocyte recovery for in-vitro fertilization. Follicular fluid concentrations of somatostatin, oestradiol, progesterone and androstenedione were measured by immunoassay. Somatostatin concentrations in concomitantly obtained plasma samples were also analysed. Follicular fluid somatostatin concentrations ranged from undetectable (< 1.5 pmol/l) to 109.4 pmol/l. The mean +/- SE somatostatin concentrations in follicular fluid (12.8 +/- 1.8 pmol/l) were significantly (P < 0.0001) increased compared to corresponding plasma concentrations of somatostatin (6.5 +/- 0.2 pmol/l). A significant and positive correlation existed between follicular fluid and plasma somatostatin concentrations (r = 0.27; P < 0.03). No differences in either follicular fluid or plasma somatostatin concentrations were found between different stimulation protocols or diagnostic groups. Neither did follicular fluid somatostatin concentration vary with follicular size. Similarly, no differences in somatostatin concentrations were found between follicular fluids associated with fertilized (13.2 +/- 2.1 pmol/l) or non-fertilized oocytes (10.5 +/- 1.6 pmol/l). Follicular fluid concentrations of somatostatin correlated positively with those of progesterone (r = 0.30; P = < 0.04), but not with those of oestradiol or androstenedione or with the androstenedione/oestradiol ratio. The relationship between follicular fluid somatostatin and progesterone concentrations suggests that follicular fluid somatostatin may have a physiological role in follicular maturation and the luteinization process.