A method for determining the individual pharmacokinetics of endogenous creatinine is derived and applied in 10 post-renal transplant patients. All patients had rapidly decreasing serum creatinine concentrations (C), but relatively constant creatinine clearance (Clcr) during the study period. Based on the multiple consecutive Clcr and the corresponding C, the elimination rate constant (K), volume of distribution (V), and daily creatinine production rate (R) for each patient were derived. The creatinine 'normal' t1/2 (corresponding to a Clcr of 120 ml min-1) was also calculated. In the 10 patients studied, the observed Clcr ranged from 29.5 to 72.7 ml min-1. The corresponding calculated mean K was 0.076 +/- 0.028 h-1; R was 20.0 +/- 5.7 mg kg-1 day-1, and V was 0.60 +/- 0.1 1 kg-1. The 'normal' t1/2 was 4.0 +/- 0.93 h. These pharmacokinetic parameters are consistent with those derived from radiotracer studies and other methods reported in the literature. The present study shows that the individual endogenous creatinine pharmacokinetics can be determined by a relatively simple and noninvasive method in certain selected patients if the appropriate serum and urinary creatinine concentrations are obtained.