1. Voltage and current recordings were made from visually identified non-pyramidal neurones in slices of layer IV of rat primary visual cortex using the whole-cell configuration of the patch clamp technique. These neurones are characterized by a high input resistance (0.5-2 G omega) and a non-adaptive behaviour of action potential frequency following depolarizing current injection, which suggests that they are stellate cells. 2. Excitatory postsynaptic currents (EPSCs) were recorded from these neurones during focal stimulation of neighbouring cells by a second patch pipette, the tip of which was placed on the soma of the stimulated cell. The response amplitude as a function of stimulus strength showed a sharp increase at a critical stimulus strength suggesting that stimulus-evoked currents represent unitary EPSCs. 3. In most cases the latencies of stimulus-evoked EPSCs were unimodally distributed with means in the range of 2.1-3.6 ms. In some experiments two peaks were seen in the distribution of latencies. The EPSC rise times, measured as the time from 20 to 80% peak amplitude, fell into a distribution ranging from 0.1 to 0.8 ms with a peak at 0.2 ms. The EPSC decay time course at -70 mV membrane potential was fitted by a single exponential with a time constant of 2.39 +/- 0.99 ms (mean +/- S.D.). The rise and decay times were independent of EPSC peak amplitudes. 4. The peak amplitude of successive unitary EPSCs, elicited by a constant stimulus, fluctuated at random. At a holding potential of -70 mV the peak amplitudes varied between 5 and 90 pA. In two out of ten cells the histogram of peak amplitudes could be well fitted by the sum of several equidistant Gaussians with a peak distance of around 10 pA. This suggests that the quantal conductance change underlying the peak current fluctuations is of the order of 100 pS. 5. At membrane potentials more positive than -70 mV the decay of stimulus-evoked EPSCs showed two components with very different time courses. In standard extracellular solution the current-voltage (I-V) relation for the fast component was almost linear whereas the slow component showed a J-shaped I-V relation with a region of negative slope conductance between -30 and -70 mV.(ABSTRACT TRUNCATED AT 400 WORDS)