In this paper, absorption and pharmacokinetic study of Radix Rehmanniae was studied by liquid chromatography coupled with mass spectrometry method after oral administration to rats. By comparing the chromatograms of ultraviolet, full scan, extracted ion and selective reaction monitoring (SRM) of standard solution, Radix Rehmanniae, blank plasma and rat plasma post drug administration, catalpol and ajugol were found to be the main compounds absorbed from Radix Rehmanniae. Plasma concentrations of aucubin, dihydrocatalpol, rehmannioside A (or rehmannioside B/ melittoside) and rehmannioside D were very low. Quantitative method for catalpol and aucubin and semi-quantitative method for other compounds in rat plasma were established. The pharmacokinetic study of those absorbed components was conducted after oral administration of 6 g x kg(-1) Radix Rehmanniae water extract to rats. Cmax, t(1/2) and AUC(0-infinity) of catalpol and ajugol were (2349.05 +/- 1438.34) and (104.25 +/- 82.05) ng x mL(-1), (0.86 +/- 0.32) and (0.96 +/- 0.37) h, (4407.58 +/- 2734.89) and (226.66 +/- 188.38) ng x h x mL(-1), respectively. tmax was at 1.00 h for catalpol and ajugol. Both catalpol and ajugol were absorbed and excreted rapidly.