The effects of dopamine-antagonistic neuroleptic (NL) medication on saccadic eye movements were compared in matched groups of 40 NL-treated and 18 NL-free schizophrenic patients and in 18 NL-treated and 14 NL-free bipolar affective patients. Manipulation of the saccadic paradigm yielded data on four types of saccade: those reflexively elicited by novel stimuli (REFLEX saccades), those directed towards the remembered location of a target now extinguished (REM) or towards the location where a predictably alternating target is expected to appear (PRED), or ANTI saccades, directed away from the stimulus to the mirror image location. Extensive psychiatric, neurological and neuropsychological assessments were also carried out on all subjects. The saccades of NL-treated patients, regardless of diagnosis, were less spatially accurate than those of NL-free patients, with a greater tendency to fall short of the target when generated towards the locus of a mentally represented target. This effect was greatest with a predictably alternating target, especially during periods when target visibility was withdrawn, only a temporal cue remaining. This pattern of impairment which is also found in early stages of Parkinson's disease is likely to be due to deficiency of striatal dopamine. Its best clinical predictors were disease duration, and Webster–Parkinsonism scores. Failure to suppress reflexive saccades to the stimulus in the REM and ANTI paradigms were more closely associated with schizophrenia than with NL treatment and were best predicted by negative symptoms and Wisconsin perseverative errors, both of which are widely regarded as indicators of frontal lobe dysfunction.