The significance of the frequency of the stimuli (target frequency) and the angular velocity of the stimuli (target speed) on horizontal optokinetic nystagmus (OKN) was investigated in 6 normal human subjects using target frequencies from 1/3 Hz to 24 Hz and target speeds from 5 degrees/sec to 120 degrees/sec. Regularity and reproductiveness of the OKN were obtained only in test conditions where each transit of targets was followed by an optokinetic response either as one beat or as a sequence of beats. This is called synchronous response and was found when the target frequency was below 3 Hz and the target speeds below 20--30 degrees/sec, depending on the actual frequency. At higher target frequencies and target speeds, the eyes were unable to take up every target, resulting in uneven responses (hyposynchronous response). Very low target frequencies and target speeds were likewise unsuitable due to drifting of the eyes from one target to another. A target frequency and a target speed close to the upper limit for synchronous response is advocated in clinical tests of OKN. 2 Hz and 20 degrees/sec is proposed as a suitable combination of frequency and target speed.