Effect of six-month two-level energy restriction, 20% or 35% restricted, on reproduction in SD rats was reported: 35% energy restriction retarted the puberty of female rats. 20% and 35% dietary energy restriction decreased the serum testosterone and luteinizing hormone concentration of two-month old rats. At the end of the sixth month, the amount of sperm and the average age of motive sperm in epididymis of energy restricted rats decreased, while the average age of abnormal sperm in epididymis increased. There was no significant difference in testis weight among three groups. These results suggest that dietary energy restriction has an unfavourable effect on reproduction in SD rats.