Spontaneous circadian fluctuations of motor symptoms in Parkinson's disease (PD) often occur, with dysfunction typically less severe in the early morning than in the afternoon. In 23 PD patients with or without a history of circadian motor fluctuations, we studied contrast sensitivity (CS), a non-motor function, considered to be dependent on dopaminergic transmission to see if it exhibits similar circadian variability. We tested CS throughout the day at 2-hour intervals beginning at 8:30 AM. To facilitate multiple testing sessions, we used a rapid, printed, forced choice test of CS not requiring a motor response. We tested CS in 43 eyes in the PD patients and 23 eyes in 12 controls at spatial frequencies of 1.5, 3, 6, 12, and 18 cycles per degree (cpd). At 8:30 AM, CS in PD did not differ from that of controls, but at all other testing times it was significantly worse at 3 or more spatial frequencies. In PD, CS was significantly worse at 2:30 PM than at 8:30 AM at 3 and 6 cpd, but in controls it was unchanged throughout the day. Separate analysis of CS in PD patients, with and without a history of circadian change in motor symptoms, revealed no significant difference between the groups. These results suggest that in PD a non-motor dopaminergic function can exhibit circadian variability and that this pattern can exist in the absence of similar variability in motor symptoms. Circadian variability which parallels the most common pattern of motor variability in PD supports the notion that the CS abnormality in this condition is related to dopamine deficiency.