Previous studies have demonstrated that the large-celled, optic pretectal nucleus lentiformis mesencephali (nLM) is essential for horizontal optokinetic nystagmus, yet little is known about its neurophysiology. In the present study, single-unit analysis of nLM utilized a large-field, patterned stimulus presented for 8 directions and 3 velocities of movement. All units localized in nLM were spontaneously active, motion sensitive, with response profiles that ranged from strongly directional and narrowly tuned to asymmetric and broadly tuned. Only about one-third of the units could be classified as directional, and no response bias for horizontal or temporal-to-nasal motion was observed. The majority of directional units showed greatest responsiveness at the lowest stimulus velocity, while the reverse occurred for many broadly tuned units. These low-velocity, highly directional units may be comparable to the ''retinal slip'' neurons recently described in the large-celled pretectal nucleus of mammals. Directional information in the nLM of Rana pipiens thus appears to be represented in the activity of a large population of motion-sensitive units which includes both narrowly and broadly tuned individual response profiles. These results are consistent with the population-coding hypothesis recently advanced to account for directional coding in other sensorimotor systems, including primate motor cortex and superior colliculus.