1. Rats were exposed to environmental temperatures of 9, 24 or 32 degrees C and the turnover of 5-hydroxytryptamine (5-HT) was estimated in various areas of brain by two different methods in separate series of experiments. In the first method, small amounts of tritium labelled 5-HT, [(3)H]5-HT, were injected into the c.s.f. in order to label radioactively the endogenous 5-HT in the brain. The rates of disappearance of [(3)H]5-HT from discrete areas of brain were taken as indices of the rates of turnover of endogenous 5-HT in these areas. In the second method, the rate of accumulation of endogenous 5-HT after inhibition of monoamine oxidase by pargyline was taken as an estimate of 5-HT turnover.2. The results obtained by each method were in agreement. The turnover of 5-HT increased with increasing environmental temperature from 9 to 32 degrees C in specific brain areas. During the first hour of exposure to 32 degrees C, the turnover of 5-HT increased in the preoptic area, hypothalamus and cortex + hippocampus, but not in striatum or midbrain. During the subsequent 2 hr at 32 degrees C, however, the turnover of 5-HT in the preoptic area and hypothalamus fell while that in the striatum showed a delayed rise.3. It is concluded that 5-HT is involved in several different cerebral pathways which increase their activity upon exposure of the rat to heat. These pathways are not located exclusively in the preoptic area and hypothalamus.