Using a polarographic method (oxygen electrode), the effects of 1−Δ⁹-tetrahydrocannabinol (an active constituent of marihuana), DL-amphetamine and pentobarbital were compared in regard to their action on oxygen consumption by mouse brain and heart homogenates. This study was unique in that the drugs were injected in vivo, while measurement of oxygen consumption was conducted in vitro for up to 8 hr. This allowed for the true active forms of these drugs—after normal biotransformation, if necessary—to exert the effects later measured. Pentobarbital was found to have no significant effect on cerebral or cardiac oxygen consumption. Both DL-amphetamine and 1−Δ⁹-tetrahydrocannabinol caused significant stimulation of oxygen consumption in the brain and heart for up to at least 2·5 hr after administration. This could indicate that both drugs cause increased synthesis and utilization of high energy compounds (i.e. ATP) via oxidative phosphorylation in the two organs. However, DL-amphetamine and 1−Δ⁹-tetrahydrocannabinol also induced limited depression of oxygen consumption in the brain and heart, respectively, at 8 hr after administration.