The ionic currents induced in cultured rat superior colliculus neurons by rapid application of glutamate (Glut) and the glutamate receptor agonists quisqualate (Quis) and N-methyl-d-aspartate (NMDA) were examined using the whole-cell patch clamp technique. Dissociated cell cultures consisting exclusively of superficial gray layer neurons from rats aged E21-P2 were used. After 7-10 days in vitro, all neurons responded to Glut and the selective agonists, NMDA and Quis. Glut was a mixed agonist, and a variable fraction (10-100%) of Glut-activated currents was due to involvement of NMDA receptors. The NMDA response was strongly regulated by extracellular Ca and Mg levels and modified by exposure to Quis. Quis transiently removed the block of NMDA-activated currents by d-amino-phosphonovaleric acid (APV). © 1987.