Synaptic transmission between pairs of neurons in layer 2/3 of in vitro slices from the rat visual cortex was studied by dual intracellular recording. The intrinsic electrophysiological properties of these neurons suggested that they were pyramidal cells. More than 1/3 of the total number of synaptically connected neurons were stained by intracellular iontophoresis of biocytin, and all had pyramidal morphology. Postsynaptic potentials (PSPs) were evoked by single action potentials (elicited by current injection) in presynaptic cells. Measurements of PSP latency, amplitude, and shape were made on spike- triggered averages. Forty-eight synaptic connections were found out of a possible total of 549, equivalent to a probability of about 0.09. For these 48 connections, the distance between the 2 impalement sites ranged from 50 to 340 microns. All PSPs were depolarizing at rest (-74 +/- 5 mV, mean +/- SD), and all 14 of the PSPs that were also recorded with the postsynaptic cell depolarized to around action potential threshold (about -55 mV) remained depolarizing. PSPs had short latencies (1.2 +/- 0.6 msec, mean +/- SD), suggesting that they were mediated by monosynaptic pathways. Peak amplitudes of the averaged PSPs varied widely (range, 0.05–2.08 mV), but the majority were less than 0.5 mV. PSPs decayed exponentially with time constants that were correlated with, but slightly longer than, the membrane time constants of the postsynaptic cells measured using injected current pulses. Four cell pairs were connected reciprocally, and 6 examples of convergent input were found in which a single cell was postsynaptic to more than 1 presynaptic cell. Trial-to-trial fluctuations in PSP amplitude were analyzed for 16 synaptic connections. The amplitude of the PSP evoked by a presynaptic action potential fluctuated more than could be accounted for by the background noise.