Circulating immunoreactive inhibin concentrations were determined in m
ale chimpanzees (n = 12) during a 36-mo period starting 18 mo before t
he onset of pubertal testicular growth (Po). Serum levels of inhibin s
tarted to increase, together with LH and testosterone, 6 mo before a r
ise in FSH and Po. Before Po, inhibin was not related to gonadotropins
but was related to testosterone levels (r = 0.263; p < 0.005; R2 = 7%
). After Po, inhibin remained related to testosterone (r = 0.289, p <
0.01) but became inversely related to gonadotropins; FSH was the best
predictor (r = -0.382, p < 0.0005; R2 = 30% ) of this relationship. D
uring the progressive rise in circulating inhibin, FSH increased and p
lateaued during 9 mo, then dropped to prepubertal levels. These findin
gs suggest that a specific closed-loop negative feedback system for th
e secretion of FSH by inhibin is not functional before the onset of pu
berty in the chimpanzee. This feedback system seems to fully operate s
everal months after the LH rise and may be related to the complete mat
uration of both Sertoli cells and spermatogenesis.