1. In anesthetized cats under partial block of nicotinic ganglionic transmission by hexamethonium and in which the cervical sympathetic trunk (CST) was split into two bundles of approximately equal size, a 40-Hz 5-s conditioning stimulus train to one bundle produced prolonged potentiation of the postganglionic compound action potential evoked by a test stimulus to the same or to the other bundle [homosynaptic and heterosynaptic, respectively, long-term potentiation (LTP)]. The LTP was detected also by recording the nictitating membrane (NM) contraction in response to a test preganglionic train. 2. The homosynaptic or heterosynaptic LTP produced by applying the conditioning 40-Hz 5-s train to one bundle was markedly depressed in amplitude and duration after stimulation of that bundle at 40 Hz for 20 min, whereas the homosynaptic or heterosynaptic LTP produced by applying the conditioning 40-Hz 5-s train to the other bundle was unchanged. The latter evidence suggests that all superior cervical ganglion (SCG) synapses can still express LTP during the depression that follows the 40-Hz 20-min train. 3. In 4 h there was no appreciable recovery of LTP from the depression produced by a 40-Hz 20-min train (n = 5). However, after 3 days (n = 3) and 5 days (n = 3), LTP recovered to 53 and 90% of control, respectively. 4. When colchicine was applied to the CST bilaterally, at a concentration sufficient to block fast axonal transport, and one CST only was stimulated for 20 min at 40 Hz, the LTP recorded 4 days later was significantly smaller on the stimulated than on the contralateral, control, side.(ABSTRACT TRUNCATED AT 250 WORDS)