The effects of taurine (2-aminoethanesulphonic acid), gamma-aminobutyric acid (GABA) and homotaurine (3-aminopropanesulphonic acid), a structural analogue of both taurine and GABA, on serum prolactin (PRL) levels were compared in conscious, unrestrained male rats. Taurine, injected into the lateral brain ventricles at doses of 6 and 10 mumol per rat, elevated serum PRL level by 52% (P less than 0.01) and 90% (P less than 0.001), respectively. GABA elevated serum PRL level (41%, P less than 0.05) only at the lowest dose (1 mumol) tested. Homotaurine was the most effective compound, eliciting increases of 353% and 449% (P less than 0.001) at 6 and 10 mumol per rat, respectively. The rank order of the three amino acids in elevating serum PRL level bears some similarity to their known rank order of potency in altering cerebral dopamine metabolism.