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Concentrations of sex steroid-binding protein (SBP) in plasma of immature male little brown bats (Myotis lucifugus lucifugus) were found to be low (15 ± 2·8 (S.E.M.) nmol/l) and stable throughout the period of hibernation which immediately precedes the onset of the first spermatogenic cycle (sexual maturity). In contrast, plasma SBP titres in spermatogenically active adults (summer) were markedly raised (238 ± 26 nmol/l). When immature males were removed from hibernation and maintained on a long photoperiod of 16 h light: 8 h darkness at a constant temperature of 25 °C to accelerate the onset of sexual maturity, these animals exhibited a rapid rise in circulating SBP. Plasma levels of SBP in immature males maintained on a long photoperiod increased significantly after 2 weeks, and by 3 weeks reached values (234± 20 nmol/l) that were indistinguishable from those seen in spermatogenically active adults. These high plasma SBP titres at 3 weeks were accompanied by increases in testicular and epididymal weights and the onset of spermatogenesis. However, no stimulation of the sex accessory glands was observed, possibly reflecting the effects of increased plasma SBP on the biological activity of circulating androgens. The ability of immature male Myotis to exhibit a pubertal increase in plasma SBP following simple environmental manipulation should provide a valuable model for the study of the control and action of sex steroid-binding protein.