The effects of protein deficiency and food restriction during pregnancy on maternal and fetal plasma and pituitary thyrotropin (TSH) levels were investigated on day 20 of gestation. The same parameters were also determined following administration of exogenous thyrotropin-releasing hormone (TRH). Maternal plasma and pituitary basal TSH concentrations were significantly elevated in protein deficiency. TRH injection to protein-deficient dams caused a marked reduction in pituitary TSH concentration, suggesting that the elevated plasma TSH seen in uninjected dams might be due to decreased metabolic clearance rather than to hypersecretion by the pituitary. Food restriction resulted in significant lowering of plasma TSH, but did not alter pituitary TSH in comparison to that of control dams. TRH administration had no effect on pituitary TSH concentration in food-restricted and control dams. Pituitaries of fetal young of protein-deprived dams were significantly smaller and contained significantly less total TSH, accompanied by a slight reduction in circulating plasma TSH. These observations are suggestive of developmental and functional retardation of the fetal pituitary. Fetal pituitary TSH was unaffected by TRH administration to control and food-restricted dams whereas, in young of protein-deprived females, total gland TSH was reduced.