1. K+ conductances were characterized in isolated, identified superior colliculus-projecting (SCP) rat visual cortical neurons. SCP neurons were identified in vitro under epifluorescence illumination after in vivo retrograde labeling with rhodamine-labeled microspheres or "beads." For experiments, SCP neurons were isolated from the primary visual cortex of postnatal day 7 to 16 (P7-P16) Long Evans rat pups after bead injections into the ipsilateral superior colliculus at p5. 2. Recording conditions were optimized to allow the characterization of Ca2+ -independent K+ conductances. SCP cells that were largely devoid of processes were selected for recording, and experiments were completed 2-30 h after cell isolation. Ca2+ -independent, depolarization-activated K+ currents were routinely recorded during 200-ms voltage steps to potentials positive to -50 mV from a holding potential of -70 mV. 3. Peak outward current densities and the relative amplitudes of the peak and plateau outward currents evoked during 200-ms voltage steps varied among SCP cells. Although cells were isolated from animals at different ages (P7-P16) and maintained for varying times in vitro (2-30 h), no correlations were found between the variations in peak current densities or peak to plateau current ratios and the age of the animal from which the cell was isolated or the length of time the cell was maintained in vitro before recording. 4. Pharmacological experiments revealed the coexpression of three K+ current components in SCP cells that could be separated on the basis of differing sensitivities to the K+ channel blockers, 4-aminopyridine (4-AP) and tetraethylammonium (TEA). Varying the concentration of 4-AP, for example, facilitated the separation of two rapidly activating K+ currents similar to A (IA) and D(ID) type currents in other cells. ID in SCP neurons is blocked by micromolar concentrations of 4-AP, whereas micromolar concentrations of 4-AP are required to effect complete block of IA in these cells. The current component remaining in the presence of high concentrations (5-10 mM) of 4-AP is slowly activating outward K+ current, similar to delayed rectifier (IK) currents in other cells. IK in SCP neurons is blocked by micromolar concentrations of TEA. 5. Activation of IA, ID, and IK in SCP neurons is voltage dependent, although the three current components display distinct time- and voltage-dependent properties. For example, although both IA and ID begin to activate at approximately -50 mV, IA activates two to three times faster than ID. In addition, the threshold for activation of IK (-30 mV) is approximately 20 mV depolarized from that of IA (or ID), and the voltage dependence of IK activation is steeper than that of IA and ID.(ABSTRACT TRUNCATED AT 400 WORDS)