The temporal organization of LH and testosterone secretion was examined in male European ferrets. Hormone levels were measured in frequent blood samples taken via an indwelling jugular cannula from sexually mature and castrated ferrets. Intact ferrets discharge LH and testosterone in discrete pulses, but the frequency and amplitude of these pulses vary within and between individual males. The average frequency of LH pulses was 1.14 +/- 0.25 pulses/h, with an amplitude of 1.59 +/- 0.23 ng/ml in 11 ferrets. Testosterone pulse frequency and amplitude were 0.62 +/- 0.04 pulses/h and 16.96 +/- 2.5 ng/ml, respectively. The frequency, amplitude, and duration of hormone pulses were similar during the light and dark phases of the light-dark cycle. LH and testosterone peaks were temporally coupled with LH pulses preceding testosterone pulses by 10-20 min. However, not all LH pulses evoked a rise in testosterone. Frequently, trains of 2 or more LH pulses gave rise to a single testosterone pulse. Castration provoked a rapid increase in the frequency of LH pulses, and the interpulse interval became strikingly uniform within hours after orchidectomy. The amplitude of LH pulses, in contrast, increased gradually over the first 6 postcastration days and then plateaued at about 4.5 ng/ml. These findings demonstrate that LH pulses constitute functionally important signals to the testis, as evidenced by temporally related increments in testosterone secretion. Moreover, distinct differences in the development of the postcastration rise in the frequency and amplitude of LH pulses suggest that testosterone operates via multiple mechanisms to regulate LH release in the adult male. Finally, this study emphasizes the utility of the ferret as an animal model to study neural determinants of LH release in the male.