We found both renal resistance to endogenous and exogenous PTH and reduced activity of the stimulatory guanine nucleotide-binding regulatory protein (Ns) of adenylate cyclase in a man with clinical signs of pseudohypoparathyroidism type Ia (PHP-Ia). Both of his children also had reduced Ns levels and short stature. The girl, 11 yr old, had evidence of partial resistance to PTH, while the son, age 7 yr, had no apparent abnormalities in calcium metabolism or response to administered PTH. Variable expression of the metabolic abnormalities of PHP during childhood has been previously described. The occurrence of reduced Ns activity in father and son is consistent with autosomal dominant inheritance for the primary biochemical defect of PHP-Ia in this family.