The effect of intermittent infusions of somatostatin (SS) on growth hormone (GH) secretion was studied in unrestrained adult male rats deprived largely of SS influence on the medial basal hypothalamus by anterolateral deafferentation (AL-cut). In addition, the influence of hypothalamic surgery on the plasma GH response to beta-endorphin (beta-END) was observed. In sham-operated rats, high-amplitude GH pulses separated by low baseline levels occurred at 185 min intervals. In rats with AL-cut, GH pulses were difficult to identify upon visual appraisal and baseline plasma GH levels became significantly higher than those of sham-operated rats. When AL-cut was performed unilaterally (half-AL-cut), low amplitude GH pulses separated by elevated baseline GH levels occurred at frequent intervals. The amount of GH secreted during 6 h was significantly reduced in rats with AL-cut or half-AL-cut as compared to that of sham-operated rats. The plasma GH response to intracerebroventricular injection of beta-END (4 micrograms) was abolished in AL-cut rats, and the response was significantly reduced in half-AL-cut rats as compared to that of sham-operated rats. When AL-cut rats were subjected to repeated infusions of SS (30 micrograms/kg b. wt./h, 150 min) separated by 30 min control periods, a large rebound of GH secretion was observed after removal and the amount of GH secreted during 6 h became comparable to that of sham-operated rats. The results suggest that SS plays important roles in the dynamic secretion of GH.