Sensitive radioisotopic enzymatic methods were used to determine 5-HT levels in 16 different regions of brain from rats anesthetized for 90-105 min with 1% halothane or 18% cyclopropane. These two anesthetics were chosen because of their differing effects on the electroencephalogram and on the cardiovascular and respiratory systems. 5-HT levels in the nucleus amygdaloideus centralis, substantia nigra, and nucleus centralis superior were increased after administration of either anesthetic, but only anesthesia with cyclopropane was associated with an increase in 5-HT level in the nucleus raphe dorsalis. The changes in levels of transmitter does not distinguish cause from effect of anesthesia, and further experiments are needed to delineate what role, if any, the specific areas play in muscle relaxation, analgesia, sleep or anesthesia.