1. The electrophysiologic properties and ionic conductances of neurons located in the stratum griseum medium (SGM) of the guinea pig superior colliculus (SC) were studied by intracellular techniques in an in vitro mesencephalic slice preparation. 2. Cells were stained with Lucifer yellow and demonstrated a uniform appearance. They had an ovoid soma with dendrites directed toward the dorsal surface. These dendrites crossed the stratum opticum, and their fine ramifications reached the stratum zonale. 3. SGM cells had a mean resting potential of 59.4 +/- 5.1 (SE) mV (n = 30), a mean slope input resistance of 26.6 +/- 10 M omega (n = 30), and a mean time constant of 4.13 +/- 1.3 ms (n = 27). 4. Direct depolarization of SC neurons produced tonic repetitive firing. These Na+-dependent action potentials showed spike-frequency adaptation. After addition of tetrodotoxin (TTX) and replacement of Ca2+ by Ba2+, slow, high-threshold spikes were also generated. The trains of Ba2+ spikes did not show adaptation. 5. In about half of the cells direct hyperpolarization elicited a slow return of the membrane potential to base line at the termination of the pulse (probably due to activation of an A-type conductance) and no anomalous rectification. The remaining cells did not have an A-type conductance but demonstrated anomolous rectification which was reversibly abolished by Cs+ but unaffected by Ba2+. 6. Some cells could be anti- and/or orthodromically activated by a stimulating electrode placed at the intercollicular commissure. These, and action potentials elicited by direct activation, had a shoulder on their falling phase. The shoulder disappeared after removal of external Ca2+ or addition of Cd2+ to the bath. 7. During repetitive firing in those cells that demonstrated an A-type conductance, the shoulder became progressively more accentuated during the train of spikes, due to inactivation of this A-type conductance. This resulted in an increase in spike duration. 8. The electrophysiological properties of these cells and their morphological characteristics suggest that they may serve as the element integrating visual and nonvisual information at the superior colliculus.