The subfornical organ of the brain has a role in the regulation of fluid balance in higher animals. In this study the effects of salt loading and water deprivation on specific proteins in this organ were investigated. For 4 days, 3 groups of rats were given an appropriate fluid diet (control, 2% NaCl and water deprived), with all groups having free access to food. Animals were killed by decapitation, and the subfornical organ was quickly dissected out and incubated for 6 h in a medium containing [35S]methionine and [35S]cysteine. Proteins from these organs were then separated by two-dimensional electrophoresis, and the resulting autofluorographs were analyzed by scanning densitometry. The results show that the incorporation of labeled amino acids into 8 proteins was changed due to the experimental manipulations.