Nine obese female patients were placed on a liquid formula diet containing 320 kcal (1.34 MJ), 31 g protein, 44 g carbohydrate, 1.5 g fat, vitamins and essential minerals for a period of four weeks under metabolic-ward conditions. The diet was tolerated well in all patients and mean weight loss was 2.49 kg per week during the four-week period. Basal serum prolactin levels were normal (2.9 +/- 0.6 ng/ml) and did not show any change during this period. In six patients a TRH test was performed before and during caloric restriction. The maximal increment of PRL after TRH was significantly lowered during dietary treatment: 53.2 +/- 16.2 vs. 32.3 +/- 11.6 ng/ml; mean +/- s.e.m.; P less than 0.025). No change was observed in the serum TSH responset to TRH before and during caloric restriction. Dietary restriction suppresses prolactin synthesis and secretion as measured in response to TRH in obese female patients.