Recent behavioral experiments examining the siphon withdrawal reflex of Aplysia have revealed inhibitory effects of strong tail shock, a stimulus commonly used as an unconditioned stimulus in studies of associative and nonassociative learning in Aplysia. We utilized a reduced preparation to perform a cellular analysis of tail shock- induced inhibition in the siphon withdrawal reflex. First, we carried out behavioral studies that showed that the reduced preparation exhibits a siphon withdrawal reflex to water jet stimuli, and that tail shock produces inhibitory behavioral effects comparable to those in the intact animal: (1) strong shock produces transient inhibition of nonhabituated responses, and (2) a habituated response is facilitated by weak shock, but not by strong shock, suggesting that increasing tail shock intensity recruits the inhibitory process that competes with facilitation of habituated reflexes. Next, we carried out cellular studies that showed that the amplitude of the complex EPSP in siphon motor neurons elicited by water jet stimuli to the siphon also exhibits the inhibitory patterns produced by tail shock: (1) the nondecremented complex EPSP (a neural correlate of a nonhabituated siphon withdrawal reflex) is significantly inhibited 90 sec after strong tail shock and recovers to preshock levels 10 min later, and (2) the decremented complex EPSP (a neural correlate of a habituated reflex) is significantly facilitated by weak shock, but is not facilitated by strong shock. In addition to the complex EPSP, we simultaneously examined the monosynaptic connection between siphon sensory neurons and siphon motor neurons. The monosynaptic EPSP does not show the pattern of inhibitory modulation by tail shock exhibited by the siphon withdrawal reflex and the complex EPSP: (1) the nondecremented monosynaptic EPSP is not inhibited 90 sec after strong shock, but tends to be above preshock levels; and (2) the decremented monosynaptic EPSP is facilitated by weak as well as strong tail shock. Our results suggest that an important component of the inhibitory process triggered by strong tail shock is mediated by neural elements presynaptic to the siphon motor neurons. Because modulation of the monosynaptic connection between identified siphon sensory and siphon motor neurons does not parallel the tail shock-induced inhibitory patterns observed in the siphon withdrawal reflex and in the complex EPSP, other synaptic connections are likely to play an important role in mediating tail shock-induced inhibition in the siphon withdrawal reflex.