The contents of serotonin (5-HT) and its major metabolite, 5-hydroxyindolacetic acid (5-HIAA), in the hypothalamus, the striatum and the remainder of the brain, after a single dose and prolonged administration of benzene were studied in rats. Benzene was injected s.c. in doses of 1/20-1/5 LD50 for single administration and in doses of 1/80-1/20 LD50 daily six days a week during four weeks of prolonged administration. Single administration produced a decrease in the 5-HT content of the hypothalamus and striatum, accompanied by an increase of the 5-HIAA level in the striatum. The highest contents of both 5-HT and 5-HIAA in the hypothalamus and the striatum were seen 45 min after benzene injection and lasted for at least 180 min. After prolonged benzene administration increases of both 5-HT and 5-HIAA contents were observed, particularly with striatum. The accelerated 5-HT turnover in rat brain after acute and prolonged administration of benzene indicates an effect of the hydrocarbon on the metabolism of 5-HT.