Seventy rats were divided into three groups (Group A, Group B, and Group C, each group being composed of two sub-groups). Group A was subjected to four retests of the Hebb-Williams test composed of 12 detour problems (Test 1, Test 2, Test 3, and Vest 4) at one month intervals, Group B was retested on Test 1 and Test 4, and Group C on Test 3 and Test 4 {Table 1). Each animal was kept an individual cage from 36 days after birth. Each animal was submitted to handling (4 days), familiarization (3 days), pretest (3 days), and the Hebb-Williams test (6 days). On the retest, the pretest and the test were repeated, 1. Group A and Group C showed significant experience effects in solving detour problems, but Group C showed no effect due to maturation. 2. Detour performance improved from trial to trial to an asymptote in all groups. 3. Neglecting the data which showed the appearance of the rats' having reached their physiological limit, no trial x test repetition interaction was observed. 4. Hypothesizing that the rat can not remember the many patterns of test and pretest for as long as one month, especially an trial 1, clear learning set curves in detour problems were obtained. 5. In the discussion, faun factors were pointed out as contributing to the experience effect; handling, reduction of exploratory drive or fear, learning set formation, and perceptual learning.