It is well-established that the activity of the endocrine thymus is under neuroendocrine control. In particular, growth hormone (GH) and thyroxine (T4) have been shown to be capable of reconstituting thymus function in hormone-deficient animals. It was therefore of interest to assess the effect of combined administration of ovine GH (0.1 mg/100 g BW/day) and T4 (10 micrograms/100 g BW/day) on serum thymulin levels in young (5 months), old (21 months) and senescent (29-30 months) male Sprague-Dawley rats. Age-matched controls received 0.1 mg bovine serum albumin/100 g BW daily during the same period (14 days). Prolactin (Prl), GH, T4 and triiodothyronine (T3) were measured in serum by radioimmunoassay, whereas serum thymulin was determined by rosette bioassay. As expected, GH and T4 were lower in the old and senescent controls whereas serum Prl displayed a slight age-related increase. No age changes were detected in serum T3. Hormone-treated animals showed supraphysiologic levels of both T4 and T3, but serum levels were comparable among the three treated age groups for each thyroid hormone. Endogenous GH levels were moderately elevated in the treated rats. In the control rats serum thymulin showed a marked reduction from 5 to 21 months of age but no further reduction was observed between 21 and 29-30 months. Hormone treatment induced a mean relative increase (% increase relative to age-matched controls) in serum thymulin of 44, 38 and 48% in young, old and senescent rats, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)