Three different pathophysiological mechanisms are probably responsible for hereditary pseudohypoparathyroidism: 1) a defect at the prereceptor-level, 2) a defective membrane N-protein accounting for diminished second messenger production, and 3) a defect in the cytosolic response to the hormone. In a cooperative, study 24 patients (mean age, 13 yr; range, 3-23 yr, 8 girls, 16 boys) receiving vitamin D metabolites (5,000-80,000 U/day) were examined and compared to a control group of 36 normal children. Immunoreactive N-terminal PTH (N-PTH), mid-C-regional PTH (mid-C-PTH), intact PTH and bio-PTH, vitamin D metabolites, and serum calcium and phosphate, alkaline phosphatase activity, and the N-protein activity of erythrocyte membranes were measured in each subject. By clinical and biochemical criteria three groups were differentiated. Eight patients had the completely expressed features of Albright's Hereditary Osteodystrophy (AHO+), including brachydactyly and/or sc calcifications, and increased N-PTH, mid-C-PTH, and alkaline phosphatase activity. Bio-PTH, intact PTH, and N-protein were normal. Nine additional patients with complete (AHO+) had elevated levels of bio-PTH, N-PTH, and mid-C PTH, normal hydroxylation of vitamin D, but decreased N-protein activity. Seven patients with pseudohypoparathyroidism had no features of AHO (AHO-), no increase of urinary cAMP excretion after exogenous PTH, normal PTH peptide levels and N-protein activity, but elevated 25-hydroxyvitamin D and decreased 1,25-dihydroxyvitamin D concentrations. In conclusion, we identified three subpopulations of PsHP: group a had a dissociation of N-PTH and bio-PTH suggesting a defective N-PTH causing renal resistance, whereas their bones respond to PTH. Group b had defective N-protein causing generalized PTH resistance. Group c was characterized by high 25-hydroxyvitamin D and relatively low 1,25-dihydroxyvitamin D levels, thus providing evidence for a defect in the cytosolic interaction of the two different second messengers for PTH, cAMP, and calcium.