To identify the developmental point of onset of learning and memory function under an operant task, performance of young rats was examined on postnatal days (PDs) 16, 19, 22, 25 and 28, and then compared with that of adult rats (PD60). Each group of Long-Evans male rats with difference days of age was assigned to a series of fixed ratio (FR) operant tasks, in which the number of lever-pressing was required for acquisition of food reward. FR rate started from 1, changed into 2, and finished with 20. Each session conducted once a day, 7 sessions per week, lasted 30 min. Response rate (the number of lever-pressing per minute) and the number of session until animal acquired the learning acquisition criterion were measured. In FR20 learning task, performance of animals at PD32 showed the same shape as those of adult rats, showing an increase of response rate and a decrease of the number of sessions. The results indicated that memory and learning function of rats required for FR20 task might be matured at PD32. Results in the present study implied that the higher brain function used for the operant FR tasks mainly depended on the hippocampal development.