Postmortem pentobarbital levels in rabbit heart blood and bone marrow were determined and compared. The average ratio of femur marrow/blood pentobarbital concentrations in 24 rabbits was 1.06 +/- 0.05. The average percent difference between actual plasma pentobarbital concentrations and calculated plasma pentobarbital concentrations was 5.82 +/- 1.96. Concentrations were determined by gas chromatography of extracted, derivatized pentobarbital.