Verhofstad et al. (Verhofstad, A. A. J., H. W. M. Steinbusch, B. Penke J. Varga, and H. W. J. Joosten (1981) Brain Res. 212: 39–49) have reported that serotonin of the rat superior cervical ganglion is contained in a distinct and separate population of small intensely fluorescent (SIF) cells. We provide evidence that the serotonin- containing SIF cells are modulated, in part, by preganglionic cholinergic neurons. For example, administration of the muscarinic agonists carbachol or oxotremorine increases the content of serotonin, and the increase induced by oxotremorine is blocked by atropine. Treatment with atropine alone or decentralization of the ganglion lowers the content of serotonin. From the rate of accumulation of serotonin in the ganglion after the administration of the monoamine oxidase inhibitor pargyline, it appears that the rate of formation of serotonin is increased after oxotremorine treatment. Reserpine, p- chlorophenylalanine, or fluoxetine treatment reduces the content of serotonin in the ganglion, suggesting that the SIF cell system has properties similar to those of serotonergic neurons of the brain. We postulate that the serotonin-containing SIF cells of the rat superior cervical ganglion participate in local circuit modulation of ganglionic transmission by receiving preganglionic information via muscarinic receptors.