Prepubertal (14 days old) and mature (10 to 12 months old) male rats were hemicastrated and killed at selected intervals during a 12-week period to compare the effect of age on acute and chronic pituitary and testicular responses to hemicastration. Testis weight was increased (P less than 0.05) in hemicastrated (HC) prepubertal rats but not in mature rats compared with intact controls. After surgery, serum follicle-stimulating hormone (FSH) concentrations were elevated (P less than 0.05) by hour 12 and day 2 in prepubertal and mature rats, respectively, and remained elevated over control levels for 4 and 8 weeks, respectively. However, the magnitude of the rise in serum FSH following hemicastration was greater in the prepubertal animals. Pituitary FSH concentrations were increased (P less than 0.05) by day 1 in prepubertal HC rats and remained elevated through week 4. In contrast, pituitary FSH levels were unaffected (P greater than 0.05) by hemicastration in mature rats. Serum concentrations of inhibin-alpha were inversely related to serum FSH concentrations in prepubertal HC rats only. Serum testosterone concentrations were reduced (P less than 0.05) following hemicastration in both age groups but recovered to control levels within 24 hours. Neither serum nor pituitary concentrations of luteinizing hormone (LH) were altered by hemicastration (P greater than 0.05) in either age group. In addition, there were no changes (P greater than 0.05) in the concentrations of testicular LH or FSH receptors following hemicastration throughout the 12-week period. However, both receptor contents were increased (P less than 0.05) in prepubertal HC rats in association with the increase in testis size.(ABSTRACT TRUNCATED AT 250 WORDS)