The development of contrast sensitivity was measured longitudinally in seven Macaca nemestrina monkeys. Operant conditioning methods were used to train and then test infant monkeys from the ages of 1 to 12 months. Several changes were observed in the contrast sensitivity function, including an overall increase in sensitivity to contrast, a shift in the peak of the function toward higher spatial frequencies, and an increase in the cutoff spatial frequency. The time-courses for the changes in the contrast sensitivity function were characterized by rapid development during the first 10-20 weeks, followed by a gradual asymptotic development to adult levels over the remainder of the year. Sensitivity to contrast was found to develop with different time-courses for different spatial frequencies; sensitivity to low spatial frequencies reached adult levels much earlier than sensitivity to high spatial frequencies.