This study was designed to determine the value of the renal clearance (CL) of N-1-methylnicotinamide (NMN), as a potential new marker of tubular function in renal impairment. Its CL was measured along with 5 other major indicators of renal function in three different models of experimental renal insufficiency in rats. Tubular and glomerular injury was induced using cisplatin and rabbit antisera to rat kidneys, respectively. For the acute model of tubular necrosis all markers were either increased or decreased maximally by the 4th day post treatment. Relative to the pretreatment values (mean values in parenthesis) the maximum reduction in CL or increase in 24h urinary excretion of the markers was: NMN 70% (17.4), creatinine 69% (4.3), urea 86% (8.9), glucose 160-fold (trace), NAG 75-fold (trace) and proteins 2.7-fold (19.7). All markers, except NMN and urea CL, fell and recovered rapidly following the injury. In the model of chronic tubular necrosis similar results were found except that the rate of decline and recovery of the markers was less pronounced. In the glomerulonephritis experiment the maximum reduction for the CL of creatinine, urea and NMN was 74%, 34%, 32% respectively. The clearance of NMN remained low for only one week and then returned to normal. It can be concluded that NMN is potentially a useful marker for proximal tubular necrosis and provides an addition to the existing battery of tests available for the assessment of renal injury.