The urinary excretion of 99mTc-mercaptotriacetylglycine (MAG3), like that of 131l-orthoiodohippurate (OIH), can be used to identify acute renal transplant rejection and measure its severity. This parameter is often quantitated as the excretory index (observed excretion/predicted excretion). A new method for predicting the urinary excretion of 99mTc-MAG3 is presented. Methods: The expected excretion was calculated from multisample plasma time-activity curves in 122 subjects, with correction for the first pass of the initial bolus. The resulting formula was tested prospectively against actual urine measurements in an additional 466 subjects. Results: Least-squares fitting led to the following equation: Predicted excretion = 0.79(1-exp(-0.0066C(MAG3)), with residual s.d. 0.06, where C(MAG3) is MAG3 clearance in ml/min and the predicted excretion is expressed as a fraction of the administered dose. Tested prospectively in the additional 466 subjects, the s.d. was 0.09. Conclusion: A new formula to predict the urinary excretion of 99mTc-MAG3 has been developed and prospectively validated. Based on our data, the normal range for the excretory index using MAC3 is the same as that of 131l-OIH, 0.8-1.2.