In the alert monkey (Macaca fascicularis) vestibular nuclei neurons and eye movements were recorded during sinusoidal optokinetic stimulation in the horizontal plane at frequencies between 0.02-3.3 Hz. Maximal stimulus velocity was generally kept constant at 40 deg/s, except for frequencies above 1 Hz. Eye movements showed a nystagmus-like pattern up to 0.2 Hz with a gain (change in eye position/change in cylinder position) greater than 0.8; at frequencies above 1 Hz the gain dropped to 0.35 at 3.3 Hz. A decrease in gain was accompanied by an increasing phase lag. Recordings in the vestibular nuclei were obtained from 'vestibular only' and 'vestibular plus saccade' neurons. Neurons with a strong eye position signal ('vestibular plus position') were excluded. The vast majority (87%) of neurons were not modulated at 0.2 Hz or higher frequencies of sinusoidal optokinetic stimulation, and were classified as 'low-frequency' type neurons. Compared to the response at constant stimulus velocity, sensitivity (imp X s-1/deg X s-1) dropped to 72% at 0.03 Hz and 16% at 0.1 Hz. A few neurons (13%) responding at 0.2 Hz (sensitivity on average 65% of the constant velocity response) were classified as 'high-frequency' type neurons. They did not respond above 1.0 Hz and showed no modulation with individual eye movements. The results suggest that the activity in the groups of vestibular nuclei neurons tested here is insufficient to account for the eye movements in response to sinusoidal optokinetic stimulation at frequencies above 0.1 Hz.(ABSTRACT TRUNCATED AT 250 WORDS)