1. Activity of single units was recorded in the flocculus of alert, behaving monkeys during sinusoidal optokinetic (0.02-5.0 Hz), constant velocity optokinetic, vestibular and visual-vestibular conflict stimulation. The maximal stimulus velocity for sinusoidal optokinetic stimulation at different frequencies was 40 deg/s or less (at frequencies above 1 Hz). For an amplitude series at 0.2 Hz, stimulus velocity was varied between +/- 10 to +/- 80 deg/s. In one trained monkey activity was also investigated during smooth pursuit eye movements and suppression of the vestibulo-ocular reflex by visual fixation (VOR-supp.). Only neurons which responded to 0.2 Hz (+/- 40 deg/s) optokinetic stimulation were included in the study. 2. The majority of neurons (44 out of 59) were type I Purkinje cells (PCs), which increased their simple spike activity during optokinetic cylinder rotation to the ipsilateral recording side. The responses during other, vestibular related, paradigms allowed all these neurons to be classified as so called 'gaze velocity' PCs. Three type II PCs were encountered, which responded similarly, but were only weakly modulated. 3. All type I PCs were modulated at frequencies of sinusoidal optokinetic stimulation between 0.05 and 2.5 Hz. PC's showed little or no modulation at 0.03 and 0.02 Hz. About half of the PC's still responded at 5.0 Hz. 4. Relative to eye velocity, the PC activity had a phase advance of about 30 deg between 0.1 and 2 Hz. It became larger at lower, and smaller at higher, frequencies. Eye velocity related sensitivity (imp/s/deg/s) was small at low stimulus frequencies and increased monotonically, on average from 0.16 at 0.02 Hz to 2.0 at 3.3 Hz. 5. Ten (out of 12) mossy fiber related input neurons were classified as visual neurons, since their activity could be related to the amount of retinal slip in all conditions. Neurons were clearly modulated at sinusoidal optokinetic stimulation up to 5 Hz. One input neuron, investigated during sinusoidal OKN, smooth pursuit eye movements, VOR and VOR-supp., behaved qualitatively like a 'gaze velocity' PC. The remaining input neuron encoded eye velocity at 0.2 Hz optokinetic, vestibular and visual-vestibular conflict stimulation. 6. The results show that during sinusoidal and constant velocity optokinetic stimulation 'gaze velocity' PC's do not encode eye velocity and/or eye acceleration. 7. The vestibular nuclei-flocculus complementary hypothesis (Waespe and Henn 1981) can explain PC responses to a large extent.(ABSTRACT TRUNCATED AT 400 WORDS)