In the nervous system of the leech Hirudo medicinalis it has been possible to study short-term plastic changes. Depression and facilitation have been demonstrated in the fast conducting system (FCS) activity; this pathway consists of a chain of electrically linked neurons present in each ganglion. In semi-intact animals or in preparation of nerve cord and segments of body wall, both electrical stimulation of peripheral roots and tactile stimulation of the skin induced, after repetitive stimulation (0.1/s) a prolonged decrement of FCS response. Strong nociceptive stimulation applied onto the head or the body wall produced a sustained facilitation of the waned response. The same potentiation has been observed by perfusing the isolated ganglion with serotonin (5 x 10(-5) M). Such a potentiation is abolished by preincubation with methysergide, an antagonist of serotonin, and with imidazole, a cAMP-phosphodiesterase activator. Such an effect is mimicked by an analog of cAMP, db-cAMP. Simultaneous recordings of both T neurons (intracellularly) and FCS firing discharge showed that, during FCS response decrement, the T cell activity remained unchanged and no modification of conductance occurred, excluding therefore a detectable involvement of sensory neurons in the depression. These results suggest that short-term plastic changes of the FCS of the leech are due to a prolonged potentiation of synaptic transmission as a result of serotonin-mediated increase in cAMP.