Bone marrow may be utilized as an alternative biological sample in cases where uncontaminated blood samples are not available for analyses. Bone marrow/blood ratios of ethchlorvynol as a function of time and dosage level were determined in 40 rabbits. A modified quantitative analysis that produced accurate and reproducible results was employed for the determination of ethchlorvynol levels. Further, ten blood and bone marrow samples containing ethchlorvynol were chosen to study the effects of storage for a period of 24 hours. Studies of blood and bone marrow ethchlorvynol levels with time showed no linear relationship. Bone marrow/blood ratios as a function of dose resulted in close mean averages with a wide range of values. Significant losses in both blood and bone marrow ethchlorvynol levels were evidenced in most of the samples subjected to the 24-hour storage study.