Oscillations with a period between 1 and 4 min have been previously observed in many visual system structures. To determine whether similar oscillations in neuronal firing also exist in rat pretectum, recordings of neuronal activity were made with standard extracellular recording methods in vivo. Oscillations with a mean period of approximately 140 s were identified in 127 recording sites in olivary pretectal nucleus (OPT). Prolonged iontophoretical current ejection of bicuculline, an antagonist of GABA(A) receptors, increased the firing rate but did not disrupt the oscillatory pattern of activity. This suggests that rhythmic activity of OPT neurons is either intrinsic to the nucleus or driven by rhythmic excitatory input. It is worth noting that oscillations within OPT were synchronized with the above-described oscillatory activity in the ipsilateral intergeniculate leaflet (IGL). In the case of simultaneous double recordings from OPT and contralateral OPT or IGL, oscillations were uncorrelated. Our findings suggest functional coupling of the OPT with ipsilateral IGL, and imply that OPT, besides its well established role in pupil constriction, might be involved in modulation of the neuronal mechanism of the circadian timing system, as was suggested previously. Alternatively, IGL might be involved in pupil diameter regulation.