Alterations of PTH secretion in patients with mild to moderate chronic renal failure were evaluated using an oral calcium suppression test. Ionized calcium and PTH were measured at 0, 30, 60, 120, and 180 min after ingestion of 2 g elemental calcium in 18 patients and 15 control subjects.The mean glomerular filtration rate was significantly lower in the patients compared to the controls (58 +/- 18 vs. 100 +/- 12 mL/min, P <0.01) but the basal ionized calcium and PTH were not significantly different. After ingestion of calcium there was a similar rise in ionized calcium with time in both patients and controls. However the mean PTH concentration in the patients was significantly higher than the controls at all equivalent ionized calcium concentrations. Overall the patients showed significantly less percentage suppression of PTH compared to control subjects, 63 +/- 10% vs. 74 +/- 9%, P <0.01. The minimum PTH value was also higher in the patients than the controls, 1.2 +/- 0.7 vs. 0.7 +/- 0.3 pmol/L, P <0.01.Thus although the majority of patients had PTH levels within the conventional reference range they demonstrated abnormal suppression of PTH secretion. The data from this study would further support the view that treatment for secondary hyperparathyroidism should be started early on in the course of chronic renal failure.