Metolazone pharmacokinetics and pharmacodynamics were investigated in five renal transplant patients and five creatinine clearance matched controls. Whereas the time to peak metolazone excretion was similar in both groups (with the exception of one renal transplant patient who subsequently was found to have chronic rejection), the percent of the administered dose excreted over 48 h was significantly less in renal transplant patients (8% in renal transplant patients vs 24% in controls). Diminished bioavailability of metolazone or tubular secretion likely accounted for this disparity in metolazone excretion. Both groups developed diuretic tolerance as indicated by hysteresis in the dose response relationship. Cumulative sodium excretion over three successive 12-h time intervals did not differ between groups. Despite this comparable natriuresis, potassium excretion was significantly less in the renal transplant patients during the first day of the study. Accordingly, metolazone administration may provide a means to "unmask" subclinical, tubular secretory dysfunction in the transplanted kidney, as exemplified by defects in metolazone secretion and potassium excretion.