We studied platelets of patients with the genetic disorder pseudohypoparathyroidism (PHP) to test whether the nucleotide-binding proteins mediating stimulation of adenylate cyclase (termed N(s)) are identical to those mediating inhibition of cyclase (termed N(i)). Functional responses to hormones that work through stimulation of adenylate cyclase are blunted in PHP patients. The erythrocytes of many of these patients (PHP-Ia) have previously been shown to have decreased N(s) activity whereas those of other PHP patients (PHP-Ib) have normal N(s) activity. We find that this decreased N(s) activity (measured by the ability to restore adenylate cyclase activity to membranes prepared from S49 cyc(-) cells) also occurs in the platelets of PHP-Ia but not of PHP-Ib patients. Platelets from both groups of patients accumulate less cAMP in response to prostacyclin than do platelets from control subjects. In contrast to the decreased N(s) function in patients with PHP-Ia, we find that N(i) function in platelets is similar in these patients and control subjects in several types of experiments: (i) epinephrine-mediated inhibition of prostacyclin-stimulated cAMP production in intact platelets; (ii) the affinity of platelet alpha(2)-adrenergic receptors for epinephrine, as determined by competition for [(3)H]yohimbine binding; (iii) the decrease in receptor affinity for epinephrine produced by Na(+) and GTP; and (iv) the concentration dependence of GTP for decreasing the affinity of these receptors for epinephrine. Because N(i) is expressed normally in platelets from patients that are genetically deficient in N(s), we conclude that N(s) and N(i) are likely to be distinct gene products.