1. The development of post-tetanic potentiation (p.t.p.) was studied at three identified synapses in the abdominal ganglion of Aplysia californica. They are (1) the excitatory synapse made onto R(15) by an axon in the right connective (RC(1)-R(15)), (2) the inhibitory synapse made by L(10) onto one of its follower cells in the rostral quadrant of the left abdominal ganglion (L(10)-LUQC), and (3) the excitatory synapse made by L(10) onto RB cells (L(10)-RB). Animals examined ranged from stage 11 juveniles weighing 2 mg to stage 13 adults weighing 300 g.2. Despite a marked decrease of the input resistance (34-fold) and a marked increase in the capacitance of the cells of the LUQC, the amplitude of i.p.s.p. measured at 50 mV more negative than its reversal potential decreases only 4-fold, between juvenile animals (25-65 mg) and adult animals (100 g). This suggests a marked increase in the synaptic current. A similar discrepancy in the extent of decrease between the input resistance and e.p.s.p. amplitude was found in the RC(1)-R(15) synapse during development.3. The amount of p.t.p., after tetanization at 5 Hz for 30 sec, increased with development. At the excitatory RC(1)-R(15) synapse, p.t.p. (defined as the percent of the maximal post-tetanus p.s.p. over the control p.s.p.) increased from 135+/-11% (mean+/-s.d., n = 7) to 334+/-72% (n = 10). At the inhibitory L(10)-LUQC synapse, p.t.p. increased from 99+/-11% (n = 6) to 276+/-58% (n = 7).4. At both types of synapses, the time course of p.t.p. decay was measured by fitting a single exponential function, showing no systematic developmental change in the time constant of p.t.p. decay from the time p.t.p. was first measured onwards. The time constant was 129+/-52 sec (n = 39) for RC(1)-R(15), and 45+/-7 sec (n = 20) for L(10)-LUQC.5. Divergent connexions made by the same presynaptic neurone L(10) on the inhibitory follower cells (L(10)-LUQC) and on the excitatory cells (L(10)-RB) showed the same amount of potentiation in the same animal after an L(10) tetanus. Convergent connexions onto the RB cells from the axon in the right connective and from L(10) produced completely independent potentiation. These results provide an independent support for the well established finding that p.t.p. results exclusively from presynaptic changes.6. At each of these several synapses which show robust p.t.p. in adult animals, p.t.p. appears as a late step in their development. There is thus a discrete and considerable interval of time between the establishment of functional connexions and the emergence of synaptic plasticity in these excitatory and inhibitory synapses in Aplysia.