We studied the in vitro influence of physiologically relevant zinc concentrations on the pituitary synthesis and secretion of prolactin (Prl). Zinc in concentrations between 1 and 10 microM reduced Prl secretion and, to a milder extent, synthesis, but not basal or stimulated growth hormone (GH) or LH release. At a supraphysiological concentration of 100 microM, zinc markedly decreased Prl synthesis and secretion, but increased LH secretion. The ability of a physiological zinc concentration to influence Prl secretion suggests that this trace element may have a role in the in vivo regulation of Prl release.