The relationships between plasma gonadotropins, testicular gonadotropin receptors, and plasma testosterone were examined during neonatal life and throughout sexual maturation in the rat. The binding affinity of testicular LH receptors (2.4 X 10(10) M-1) was significantly higher than that of FSH receptors (2.1 X 10(9) M-1) at all stages of development. The concentration of FSH receptors in the testis reached a peak between 10-15 days of age, then fell to a constant level from 25-90 days. However, the testis content of FSH receptors increased continually with age and reached a plateau at day 60. Plasma FSH declined after birth to a nadir at 15 days, then rose rapidly to a peak at day 38, and fell to a plateau from day 50 through adult life. In contrast to the rapidly changing profile of plasma FSH during early maturation, alterations in plasma LH were less marked throughout development. Although a progressive rise in plasma LH concentration was observed between days 36-51, the simultaneous changes in testicular LH receptors and plasma testosterone were much more prominent. Testicular LH receptors showed a continuous increase in concentration and total number with advancing age and testis growth. The major rise in LH receptor concentration occurred between 15-38 days age, at the same time as the rise in plasma FSH concentration and the phase of rapid testicular growth. Plasma testosterone fell during the 8th-24th days after birth, then rose rapidly between days 35-55. The pubertal rise in plasma testosterone occurred about 15 days after testicular LH receptors began to increase and was coincident with the continuing rise in LH receptor content from day 35 until day 55 and with the progressive increase in plasma LH during this period. These observations have demonstrated that the early development of testicular FSH receptors in followed by a prominent rise in plasma FSH, with concomitant increases in testicular growth and LH receptor concentration. The resulting increase in gonadal sensitivity to LH could be responsible for the marked increase in secretion of testosterone which occurs during puberty in the presence of a relatively small change in the circulating LH concentration. The sequence of changes observed in gonadotropins and their testicular receptors is consistent with the view that FSH-induced testicular sensitivity to LH is an important factor in sexual maturation in the male rat.