Luteinizing hormone stimulates testicular Leydig cells to produce testosterone by binding to a receptor that activates the G protein Gs and adenylyl cyclase. Testotoxicosis is a form of precocious puberty in which the Leydig cells secrete testosterone in the absence of luteinizing hormone, often due to constitutive activation of the luteinizing hormone receptor and (indirectly) Gs (refs 1-4). Here we study two unrelated boys suffering from a paradoxical combination of testotoxicosis and pseudohypoparathyroidism type Ia (PHP-Ia), a condition marked by resistance to hormones acting through cyclic AMP (parathyroid hormone and thyroid-stimulating hormone) as well as a 50% decrease in erythrocyte Gs activity (the remaining 50% is due to the normal Gs allele). In both patients, a mutation in the gene encoding the Gs alpha-subunit replace alanine at position 366 with serine. We show that this alpha s-A366S mutation constitutively activates adenylyl cyclase in vitro, causing hormone-independent cAMP accumulation when expressed in cultured cells, and accounting for the testotoxicosis phenotype (as cAMP stimulates testosterone secretion). Although alpha s-A366S is quite stable at testis temperature, it is rapidly degraded at 37 degrees C explaining the PHP-Ia phenotype caused by loss of Gs activity. In vitro experiments indicate that accelerated release of GDP causes both the constitutive activity and the thermolability of alpha s-A366S.