Inhibins and activins are implicated as endocrine regulators of follicle-stimulating hormone production and of testicular steroidogenesis and spermatogenesis in mammals. The potential involvement of these proteins in cockerels was investigated by measurement of circulating inhibin A, inhibin B, total inhibin alpha-subunit immunoreactivity (ir-alpha), activin A, LH, FSH, and testosterone from the juvenile state through to sexual maturity. Plasma inhibin A remained low between 6 to 12 wk of age and increased approximately threefold (P < 0.05) to a prepubertal peak between Weeks 14 to 18, followed by a gradual decline to the end of the study (Week 24). Although plasma FSH levels were not correlated to inhibin A before Week 16 (r = -0.17), they were negatively correlated from Week 18 (r = -0.49; P < 0.005). Inhibin B levels were below the assay detection limit until 16 wk of age but thereafter rose steadily in parallel with FSH (r = 0.27; P < 0.02) and testosterone (r = 0.35; P < 0.005). Thus, inhibins A and B showed divergent profiles during sexual maturation. Plasma ir-alpha levels were much higher than dimeric inhibin levels throughout, although the relative difference varied with age. Plasma activin A levels were below the assay detection at all times. Juvenile cockerels were actively immunized against a synthetic chicken inhibin alpha-subunit peptide conjugate to determine effects on plasma hormones and on testicular weight, morphology, and activin A content. Immunization generated circulating antibodies that bound (125)I-bovine 32-kDa inhibin but did not affect plasma FSH or testosterone levels at any stage of development. However, immunization reduced postpubertal plasma LH levels (P < 0.05) and promoted increased testicular weight (24%; P < 0.01) and total testicular activin A content (42%; P < 0.001) at 24 wk. Testis weight of immunized birds was positively correlated with inhibin antibody titer (r = 0.61; P < 0.05). Live weight gain was not affected by immunization. Morphometric analysis of testis sections showed that inhibin immunization had no effect on the fractional volume of the seminiferous tubule wall, seminiferous tubule lumen, or interstitial tissue area. Likewise, seminiferous tubule surface area and surface area:volume ratios were not different from controls. These findings support differential roles for inhibins A and B in regulating the pituitary-testicular axis during sexual maturation in the cockerel but highlight the need for more detailed studies to distinguish between potential endocrine and local intragonadal roles of inhibin-related peptides and to elucidate the mechanism by which immunization against inhibin alpha-subunit promotes testis enlargement without raising plasma FSH.