Field potentials and radial current source-densities (CSDs) evoked by optic tract stimulation were observed in goldfish tectum in vitro. The effects of different concentrations of calcium and magnesium were studied to improve understanding of component events in retinotectal transmission and interpretations of effects of pharmacological agents. Responses were of 3 forms: 'non-synaptic', 'subthreshold', and 'complex.' Subthreshold responses occurred when amplitudes were less than 40% of their maximal levels. They consisted of a sink-source pair with the sink in the superficial optic neuropil and the source 100-150 micron deeper. They were monophasic, rising in 1-2 ms and decaying as simple exponentials with time constants between 3.3 and 4.5 ms. Several other conditions which reduce amplitudes (besides low [Ca2+] and/or high [Mg2+]) produce responses of the subthreshold form. Complex responses, observed when amplitudes were 40-100% of maximal, were characterized by more rapid rise and decay and included, in the most extreme cases, a late, long, low-amplitude sink-source pair of opposite polarity. We propose that the time course of decay of subthreshold responses is determined by the passive cable properties of bi- or multistratified neurons with one dendritic arbor in the optic neuropil and a second arbor 100-250 micron deeper. Complex responses probably include recurrent inhibition to depolarized dendrites of the optic neuropil, latent by 4 ms to the monosynaptic excitation. Pharmacological assessments of retinotectal transmission may be made more precise by using low [Ca2+]/high [Mg2+] media to isolate monosynaptic activity.