Pseudohypoparathyroidism type I is a hereditary disorder characterized by resistance to parathyroid hormone and other hormones that work via cyclic adenosine 3',5'-monophosphate (cAMP). Most patients with this disorder have generalized deficient activity of Ns-protein (type Ia) which couples stimulatory hormone receptors to catalytic adenylate cyclase. In patients with normal Ns-protein activity (type Ib), a decreased incidence of resistance to hormones other than parathyroid hormone exists. Mental deficiency of unknown cause occurs in 47% to 75% of all patients with pseudohypoparathyroidism type I. Because mutations in the adenylate cyclase-cAMP system may affect the learning ability of Drosophila flies, we assessed mental deficiency in 25 patients whose Ns-protein activity we had determined: 9 of 14 patients with type Ia disorder and 0 of 11 patients with type Ib disorder had mental deficiency. We suggest that Ns-protein deficiency, reduced cAMP levels, or both, are involved in the mental deficiency in these patients and probably in mental function in humans.