Two methods for assessing 5-hydroxytryptamine (5-HT) turnover rate have been tested in the cerebral vessels of the rat. The pretreatment of the animals with benserazide 45 min before death brought about an increase in the levels of 5-hydroxytryptophan (5-HTP). When the MAO inhibitor pargyline was injected and the animals killed at different times, there was an exponential decrease in the concentration of 5-hydroxyindole acetic acid (5-HIAA) with time, whereas the increase in 5-hydroxytryptamine was linear only during the first 30 min, thereafter reaching a plateau. This pattern was similar to that obtained in the caudate nucleus after MAO blockade. In the hippocampus, pargyline induced a lineal accumulation of 5-HT throughout the experiment as well as an exponential decay of the 5-HIAA concentration. These results indicate that the turnover rate of 5-HT can be appraised in the rat cerebral arteries either after the rate of disappearance of 5-HIAA or from the accumulation rate of 5-HTP.