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Male rats aged 30, 60 or 125 days were injected once every 5 days with 5 mg of a long-acting testosterone preparation, or its vehicle, for periods of 5–40 days. Pituitary and serum follicle-stimulating hormone (FSH) levels were determined by bioassay and radioimmunoassay respectively. Treatment with testosterone produced a consistent pattern of change in pituitary FSH levels (compared with control animals) over a period of 35 days: an initial increase in relative FSH levels was followed by a sharp drop after which there was a further increase in content. However, the magnitude and rate of these changes varied according to the age of the animals involved. These changes in pituitary FSH levels are considered to reflect mainly changes in synthesis, as secretion of FSH was apparently reduced at all ages in testosterone-treated animals. The age-dependent changes in FSH synthesis in testosterone-treated animals may reflect a decrease in sensitivity to testosterone feedback with increase in age. Moreover, in untreated male rats the decrease in FSH secretion after 35 days of age and the apparent reduction in FSH synthesis after 76 days of age could be accounted for by changes in serum testosterone levels.