Prepubertal (16 weeks old) ram lambs were used to investigate the effects of hyperthyroidism and hypothyroidism on development of reproductive endocrine function. Over a period of 8 weeks, ram lambs were made hypothyroid (serum T4 less than or equal to 3 ng/ml compared with controls congruent to 30 ng/ml) by daily oral administration of methyl thiouracil or hyperthyroid (serum T4 congruent to 135 ng/ml) by daily sc injection of T4. Hyperthyroidism was associated with decreases in LH pulse frequency (2.25 +/- 0.75/12 h compared with controls 5.75 +/- 0.48/12 h), basal LH, and mean LH concentrations, together with arrested testicular growth and aspermatogenesis. Hypothyroid rams showed normal pubertal development. After iv injection of LHRH, hyperthyroid ram lambs showed similar LH responses to control and hypothyroid rams. Basal testosterone production (5.6 +/- 1.0 ng/min) and plasma testosterone concentrations after human CG (2.0 +/- 0.7 ng/ml) in hyperthyroid rams were significantly lower than in controls (67.5 +/- 24.0 ng/min and 5.2 +/- 1.0 ng/ml, respectively). It is concluded that retarded testicular development in hyperthyroid ram lambs results from changes in hypothalamo-pituitary activity manifested in a decreased LH pulse frequency.