The distribution of radioactivity in rats after administration of C14-labeled methadone.

1. Methadone HBr labeled with C14 in the 2-position proved to be equally effective as an analgetic in rats (10 mgm./kgm.) as the unlabeled drug. 2. Tissue specific activity values appear to be comparable to methadone concentrations. The possibility that methadone is conjugated in the body has not been completely excluded. 3. Methadone HBr is rapidly and completely absorbed after administration by the subcutaneous route. 4. At periods of from 1 to 3 hours after administration, the specific activity of brain was no higher than carcass levels, therrby indicating no particular affinity of central nervous system tissue for methadone. Concentrations well above carcass levels were found in the adrenals (at 1 hour), thyroid, lungs, spleen, kidneys, liver, and gastro-intestinal tract. 5. Methadone HBr is almost completely excreted after 24 hours. Excretion is primarily via the intestinal route and secondarily via the kidneys.