In a study to test the hypothesis that hyperprolactinemia is caused by subclinical zinc deficiency, dark-adaptation curves of seven women with primary hyperprolactinemia and seven normal women were measured to assess tissue zinc status. Although plasma zinc levels, final dark-adapted thresholds, and the time courses of rod dark-adaptation did not differ significantly between patients and normal subjects, the median cone plateau of the hyperprolactinemic patients was significantly higher (0.66 log units) than that of normal subjects. It appears unlikely that derangements of vitamin A metabolism, for which zinc is a cofactor, explains this unanticipated and subtle abnormality in dark adaptation of the hyperprolactinemic women.