The aim of this study was to investigate the involvement of somatostatin (SRIF) in the thyrotroph adaptation to nutritional changes. For this purpose, we studied the effects of passive immunization with SRIF antiserum (A-SRIF) on the reduced basal TSH secretion in rats starved for 72 h and on the plasma TSH surge following carbohydrate (CHO) refeeding. This latter experiment was performed at two different times of the day in order to elucidate whether SRIF may participate in the regulation of the plasma TSH circadian rhythm. In chronically catheterized rats, we observed that A-SRIF injection induced a similar pattern of plasma TSH rise over a sampling period of 5½ h in both fed and rats starved for 72 h (3-way analysis of variance). In morning experiments, CHO refeeding or A-SRIF injection elicited a significant rise in plasma TSH. The amplitude and duration of the response was proportional to the injected dose. In evening experiments, although basal TSH values were significantly lower than those observed in the morning ones, maximal plasma TSH values after A-SRIF injection were not significantly different. At both times of the day, association of refeeding and A-SRIF injection did not stimulate TSH further than either refeeding alone or A-SRIF alone. In conclusion, our data suggest that SRIF (1) cannot account for the differences in serum TSH levels between fed and starved rats; (2) is not responsible for the diurnal difference in basal serum TSH in starved rats, and (3) seems to be involved in the TSH response to refeeding