analysis of ritonavir in plasmaserum and tissues by highperformance liquid chromatography a method has been developed to quantify ritonavir concentrations in human plasma and in mouse serum liver and brain using highperformance liquid chromatography extraction recoveries for ritonavir and its internal standard averaged greater than 95 withinday variability expressed as a coefficient of variation averaged 6 over the concentration range 05 microgml to 15 microgml ritonavir and betweenday variability averaged 56 over 5 microgml to 15 microgml ritonavir the method was applied to quantitation of ritonavir in mouse serum and tissue measured values deviated less than 5 from the actual values in mouse serum liver and brain samples containing 5 microgml ritonavir the slopes of calibration curves for extracted calf serum mouse serum mouse liver and mouse brain standards were nearly identical to the calibration slope of standards which were not extracted all curves were linear through zero and r2 was no less than 0998 for any form of calibration in addition there was no chromatographic interference from commonly prescribed medications