One isotopic and three non-isotopic methods were used to determine the effect of an acute intravenous dose of delta 9-tetrahydrocannabinol (delta 9-THC, 2 mg/kg) on the rat brain turnover rate of serotonin. 2. In control animals the turnover rate of serotonin was about 2 nmol/g per h. This rate was not altered by delta 9-THC when it was calculated from the rise of 5-hydroxyindoleacetic acid following probenecid from the rise of serotonin following pargyline. 3. delta 9-THC did not alter serotonin turnover rate when it was calculated from the conversion of 3H-tryptophan to 3H-serotonin. 4. The serotonin turnover rate was significantly increased by delta 9-THC when the rate was calculated from the decline of 5-hydroxyindoleacetic acid following pargyline. 5. These results suggest that delta 9-THC does not alter the turnover of rat brain serotonin. The previously reported delta 9-THC-induced changes in body temperature and increased brain levels of 5-hydroxyindoleacetic acid may be mediated by some other mechanism such as interference by delta 9-THC of the vesicular binding of serotonin.
