In the present study we examine the developmental appearance and maturation of synaptic plasticity at the A-B neuron synapse in the cerebral ganglion of Aplysia. In the CNS of juvenile Aplysia 120 days after hatching, the excitatory synaptic connection between A and B cluster neurons is essentially the same as in the adult cerebral ganglion. No differences were observed between the amplitudes of the initial EPSPs in the cerebral ganglia of juveniles and adults. One form of plasticity, low frequency synaptic depression, is also present in juveniles. Another form of activity-dependent plasticity, slow developing potentiation (SDP) appears and matures during the late juvenile stage of development. At 120 days posthatching SDP, evoked by tetanic stimulation, is largely absent. Potentiated EPSPs have a significantly smaller amplitude than in adults. Over the next 80 days SDP undergoes a maturation process. The peak potentiation increases linearly with age from 135 +/- 12% at 125 days to 275 +/- 20% at 188 days. The duration of the potentiation, as measured by the time-constant its decay, also increases linearly from 12.7 +/- 3.4 min to 27.9 +/- 3.9 min. From 120-170 days, < 50% of the A neurons tested exhibited SDP. After 170 days, > 85% exhibited SDP. Changes in the rising phase of the A neuron action potential have been implicated in mediating SDP. At 120 days, the A neuron action potential has a significantly shorter duration (half-width) than in the adult. Between 120 and 200 days, both the duration and rise-time of the A neuron action potential increase linearly. These results confirm findings by other investigators, that different forms of synaptic plasticity develop independently, with depression appearing before potentiation.