Outpatient renal stone formers belonging to the established urolithiasis subgroups and controls were examined with respect to urinary and serum citrate (Cit) and several associated variables. Only in the normocalciuric majority of calcium and in uric acid stone formers was Cit in 24-hour urine decreased, but was normal in 2-hour fasting morning, and in 3-hour postprandial urine following a Cit-free test meal. Serum Cit was elevated in normocalciuria, renal and resorptive hypercalciuria. This Cit constellation was associated with either normal (absorptive, renal hypercalciuria) or low (normocalciuria, uric acid stone formers) parathyroid gland function as assessed by serum parathyroid hormone and nephrogenous urinary cyclic AMP, except in patients with primary hyperparathyroidism. In 2-hour morning urine the magnesium/creatinine ratio (normocalciuria) and ammonia excretion (uric acid stone formers) were decreased, while ammonia in 24-hour urine was low in all stone formers. It is suggested that Cit metabolism is altered in renal stone disease in general, and that in normocalciuria, stone inhibitors (Cit; magnesium) may be deficient.