Resting human muscle blood flow (MBF) was determined simultaneously in the usual intramuscular injection sites to resolve whether variance in MBF could account for differences in drug absorption. Three pairs of muscles (gluteus maximus, vastus lateralis, and deltoid) were studied in each of 20 adult subjects. Use of dual, matched linear rate meters allowed two muscles to be studied simultaneously, with the order of injection random within an incomplete block design. MBF was calculated from the 133xenon washout rate using a single exponential that the computer found to best fit the data. Deltoid MBF (11.6 ml/100 gmlmin ± 0.5) was significantly (p < 0.05) greater than gluteus MBF (9.6 ± 0.5), with vastus MBF intermediate (10.8 ± 0.6). There were no significant differences due to order of injection or between right and left sides for each muscle. These data indicate that there are consistent differences in resting MBF among specific muscle groups of sufficient magnitude (19%) to affect the rate of absorption and peak serum levels following intramuscular administration of drugs.