In the present study, antidromically activated raphe-spinal neurons in the nucleus raphe magnus are found to exhibit a wide range of conduction velocities and spontaneous discharge rates. The unit activities observed in control rats have been compared with those in 5,6-dihydroxytryptamine (a specific neurotoxin to serotonergic cells)-pretreated animals. It is found that the units conducting at slower than 12 m/sec and discharging at rates of less than 3 Hz often found in control rats are not found in drug-pretreated animals, and are thus presumed to be serotonergic. In addition, the regularity of spontaneous discharges of units has been evaluated by testing the coefficient of variability of spike intervals. These results indicate that the slow-conducting units discharge less regularly than the fast-conducting ones.