The capacity of several potent pharmacological agents to increase growth hormone (GH) release was tested in young (3- to 4-month-old) and old (18- to 20-month-old) male Sprague-Dawley rats. Young and old animals were injected with alpha-methyl-p-tyrosine (250 mg/kg), a catecholamine synthesis inhibitor, followed 60 min later by the alpha-adrenergic receptor agonist, clonidine hydrochloride (150 micrograms/kg). Plasma GH levels increased to greater than 276 ng/ml by 30 and 50 min in young rats, whereas values never exceeded 130 ng/ml (p less than 0.01) in old rats. Piribedil methane sulfonate (1 mg/kg), a dopamine receptor agonist, significantly increased GH in young animals 60 min after injection (p less than 0.01), but had no effect in old rats. Morphine sulfate (5mg/kg) increased plasma GH concentration in young rats to greater than 400 ng/ml at 30, 50 and 70 min after injections, whereas values in old animals never exceeded 300 ng/ml (p less than 0.01). 15 min after injection of somatostatin antiserum No. 774 (5 ml/kg), plasma GH levels increased to greater than 750 ng/ml in both young and old rats. Administration of greater volumes of somatostatin antiserum (8 ml/kg) increased GH levels more in old than in young animals 30 and 45 min after injection (p less than 0.01). These results indicate that old male rats have less capacity to increase GH in response to agents that normally enhance GH release. This may be the result of a diminished number of affinity of post-synaptic neurotransmitter receptors, or to an increased release of somatostatin. Since somatostatin antiserum in creased H equally or to a greater extent on old that in young animals, aging rats may release more somatostatin or the pituitary may be more sensitive to the inhibitory effects of this hormone.