From each of 14 women undergoing laparotomy at different stages of the luteal phase of the cycle, the corpus luteum (CL) was excised, cut into pieces and incubated for 120 min. Incubations were performed in the absence and presence of hCG and prostaglandin F2 alpha (PGF2 alpha). In addition, noradrenaline (NA) was added to the incubation medium of newly formed CL, and the adrenergic blocker propranolol to incubated specimens of mid-luteal phase CL. After incubation the tissue levels of protein and cyclic AMP (cAMP) and the media concentrations of progesterone (P) were determined. In young CL, PGF2 alpha did not interfere with the stimulatory effect of hCG on neither cAMP nor P formation, while in CL of the mid-luteal phase PGF2 alpha significantly counteracted the stimulatory effect of hCG on these two parameters. In the presence of propranolol, however, this antigonadotrophic effect of PGF2 alpha in vitro was abolished. Furthermore, when NA was added to specimens of young CL, PGF2 alpha significantly counteracted the stimulatory effect of hCG on both cAMP and P formation. These in vitro data show that endogenous catecholamines may regulate the antigonadotrophic, 'luteolytic' effect of PGF2 alpha in human CL.