We have tested the hypothesis that physiological puerperal hyperprolactinaemia may be secondary to reduced hypothalamic dopaminergic inhibition of prolactin (PRL) release. Nine post-partum females with physiological hyperprolactinaemia (aged 19-40 years; mean basal PRL +/- SE, 2099 +/- 257 mU/1, range 1002-3762 mU/1) were studied and results compared with fourteen normoprolactinaemic females (basal PRL less than 400 mU/1; aged 18-36 years). Puerperal hyperprolactinaemic females showed a reduced TSH response to dopamine (DA) receptor blockade with metoclopramide (10 mg i.v.) compared with normal females over a 60-min period following drug administration (total incremental TSH change, mean +/- SE, mU/1; 0.5 +/- 0.3 v. 3.1 +/- 1.0 P less than 0.005). This finding of reduced dopaminergic inhibition of TSH release in females with physiological puerperal hyperprolactinaemia supports the view that an overall reduction in hypothalamic dopaminergic tone may contribute towards hyperprolactinaemia in post-partum women.