Pain modulating neurons of the rostral ventromedial medulla (RVM) include three physiologically distinct classes of neurons in intact, anesthetized animals: and cells that change their activity before the onset of withdrawal reflexes and cells, which have activity unrelated to withdrawal reflexes. A previous in vitro intracellular study demonstrated that the RVM contains two types of neurons that are distinguished by their action-potential characteristics. The present in vivo intracellular study examined whether these intracellularly recorded action-potential characteristics are correlated with the physiological response properties of RVM neurons recorded. RVM neurons exhibited two distinct types of action potentials in vivo. Fast-spike (FS) neurons (n = 30) had short-duration action potentials (0.27 +/- 0.02 (SE) ms at half amplitude) and biphasic afterhyperpolarizations with a characteristic rapid overshooting spike repolarization. Slow-spike (SS) neurons (n = 25) had longer duration action potentials (0.44 +/- 0.02 ms at half-amplitude) due to a slower-spike repolarization rate and monophasic afterhyperpolarization. and cell classes included both FS and SS neurons. FS and neurons had an early onset response to noxious heat stimulation. SS and cells showed a delayed onset response to noxious heat. cells (n = 13) were all SS cells. Among the SS neurons, only cells had action potentials longer than 0. 45 ms (n = 9). FS and SS neurons were intermingled throughout the RVM. The majority of intracellularly labeled cells (n = 15) had fusiform somata with two to five fine caliber primary dendrites and a predominantly mediolateral orientation of the long axis of their dendritic tree. All labeled FS cells (n = 5) had large, multipolar somata with four to nine large caliber primary dendrites. The present study defines in vivo membrane and morphological characteristics of RVM neurons that correlate with physiological differences and may be used for identification of nociceptive modulatory RVM neurons in slice preparations.