Various in vitro methods employing radioactive indicators have been presented for the determination of glomerular filtration rate (GFR) including slope/intercept (S/I) and single sample (SS) methods. The S/I technique utilizes the analysis of the slope of a certain segment of the plasma disappearance curve of a suitable radioactive indicator and its intercept on the y axis. The SS method is based on the theoretical volumes of distribution derived from a single plasma concentration at some specific time after intravenous injection of the indicator. Using GFRs estimated from the compartmental analysis of the entire plasma disappearance curve as a reference standard, we have compared the errors of GFR estimation calculated from 12 S/I methods, 3 SS methods and 2 techniques in which original formulae were derived. The errors of the SS method of Tauxe et al. gave the lowest standard error of estimate (Sy X x) of all the methods. The S/I methods gave lowest errors when sampling times were taken between 60 min and 240 min. Almost all the errors observed here were significantly less than those observed using scintillation camera techniques. The SS method seems to be the method of choice for estimation of GFR by single injection techniques. The use of the scintillation camera in conjunction with the SS technique would provide a useful right to left ratio so that individual kidney GFR could be calculated.