The timing of puberty appears to be affected by photoperiod to a greater extent in female lambs than in male lambs. Melatonin, a daylength information conveyor, shows the same plasma levels in female lambs as in male lambs. We therefore tested the hypothesis that the predominant melatonin binding sites, located in the pars tuberalis (PT) of the pituitary, might display sexual dimorphism. Here, in two independent experiments, we report no difference in quantity, density or dissociation constant between female and male peripubertal lambs' PT melatonin binding sites, measured by individual saturation curves. In October, Bmax values were 19.5 +/- 1.8 and 19.6 +/- 1.2, whereas in June, they were 94.1 +/- 8.6 and 96.4 +/- 12.1 fmol/mg protein (mean +/- S.E.M.) for females and males, respectively. The Kd values were in the range of 10-20 pmol. The sexual parameters measured, i.e. presence of corpus luteum in female ovaries or testis weight in male lambs, were not linked with PT melatonin binding numbers.