1. Heterosynaptic facilitation, defined as an increase of the efficacy of synaptic transmission between a test interneurone and a post-synaptic neurone, produced by the stimulation of a separate pathway, was studied in the left pleural ganglion. The experimental procedure consisted of detecting the effects of a brief tetanus, applied to tentacular and tegumentary nerves, on the amplitude of monosynaptic and unitary post-synaptic potentials (p.s.p.s) recorded in the left giant cell and generated by stimulating the test interneurone every 10 sec. The membrane potential of the test interneurone was simultaneously recorded. 2. Following heterosynaptic stimulation, the amplitude of the test p.s.p. increased, after a delay of about 30 sec, up to 250% of its original size; this increase subsided after 2-3 min or more. 3. Only the interneurones producing in the giant cell the e.i.p.s.p. (excitatory-inhibitory post-synaptic potential) were affected by hetero-synaptic facilitation. Other interneuronal types showed no changes in their synaptic transmission on the giant cell after heterosynaptic stimulation. 4. Heterosynaptic stimulation did not produce either orthodromic or antidromic spikes in the test interneurones clearly indicating that facilitation of test p.s.p. did not result from increased spike activity in the test interneurone. 5. Often heterosynaptic facilitation of the test p.s.p. was observed due to spontaneous activity in the heterosynaptic pathway, demonstrating the normal occurrence of the phenomenon. 6. Iontophoretic injection of 5-HT at critical, presumably synaptic, sites in the neuropil, evoked a facilitation of the test p.s.p. similar to heterosynaptic facilitation. Only the e.i.p.s.p.s. were so affected by 5-HT. On the contrary, other p.s.p. types were depressed by 5-HT as a result of conductance changes in the left giant cells. 7. Both heterosynaptic facilitation and 5-HT facilitation were suppressed by the presence in the bath of 5-HT (10(-5) M) and of LSD-25 (3 X 10(-4) M). The action of injected 5-HT on the membrane conductance of the left giant cell was also depressed in the pressence of 5-HT in the bath, but was unaffected by LSD-25 (3 X 10(-4) M). 8. From the parallelism of properties of heterosynaptic and 5-HT facilitation, it is suggested that 5-HT is the probable transmitter mediating heterosynaptic facilitation. It seems likely that 5HT is released from the heterosynaptic pathway at the level of the synaptic ending of the test interneurone on to the giant cell and that it increases the efficacy of this synapse, probably acting on the quantity of synaptic transmitter liberated.