We used multiple-site optical recording methods, in conjunction with impermeant molecular probes of the cell membrane potential, to record the electrical activity of model neural circuits in vitro. Our system consisted of co-cultured pairs of left upper quadrant neurons from the abdominal ganglion of the marine gastropod Aplysia. These neurons interact via inhibitory synapses in vitro. Photodynamic damage to the neurons was essentially eliminated over the time course of the measurements, approximately less than 30 s, by removing oxygen from the recording solution and replacing it with argon. This procedure did not affect the synaptic interactions. We observed repetitive spiking activity in single-trace optical recordings with a maximum signal-to-noise ratio per detector of approximately 50. Individual optical signals that corresponded to either the activity of the presynaptic neuron or that of the postsynaptic neuron were clearly identified. This allowed us to monitor the activity of synaptically interacting neurons, observed as a reduction of the firing rate of the postsynaptic cell after activity of the presynaptic cell. Our results demonstrate that optical methods are appropriate for recording prolonged, asynchronous activity from synaptically interacting neurons in culture.