1. The sensorimotor synapse of Aplysia expresses various shortlasting changes in synaptic efficacy including homosynaptic depression (HSD) and heterosynaptic facilitation by serotonin (5-HT) either at nondepressed sensory neuron (SN) synaptic connections or at SN synaptic connections first depressed by HSD. We examined the temporal sequence of expression for these three forms of synaptic plasticity as synaptic connections between SN and target motor cell L7 were reestablished and stabilized in cell culture. The same cultures were reexamined at different time points. 2. We found that only HSD and facilitation of nondepressed synapses were expressed at "mature" levels on day 1 in culture, whereas facilitation of depressed connections was significantly weaker on day 1 than the facilitation evoked on day 4. 3. The late expression of 5-HT facilitation of depressed SN synaptic connections was not a result of a reduced capacity of two kinases activated by 5-HT (protein kinase A and protein kinase C) to evoke facilitation. Direct activation of the kinases with either cyclic AMP or phorbol esters evoked the synaptic facilitation both on day 1 and day 4. 4. The late expression of 5-HT facilitation of depressed SN synaptic connections was correlated with the late functional expression of receptors sensitive to 5-HT antagonists cyproheptidine or methiothepin. Both antagonists significantly interfered with 5-HT facilitation on day 4, but both had little effect on 5-HT facilitation of the same cultures examined on day 1. 5. Unlike the properties of SNs in the intact nervous system, both antagonists reduced significantly the excitability changes evoked by 5-HT when the SNs were plated either alone or with target cell L11 that fails to induce synapse formation. When cultured with L7, however, both antagonists evoked little change in 5-HT excitability. In the presence of L7, the SNs expressed the phenotype more typical of SNs in the intact nervous system. 6. The results suggest that target interactions not only influence the formation of chemical connections but they also may regulate the acquisition of specific plastic properties by the presynaptic neuron including the functional expression of receptors for neuromodulators.