Recent reports indicate that weight cycling (repeated periods of weight gain and loss) cause an organism to become an energy conserver, meaning that the organism gains weight more quickly and loses weight more slowly during subsequent weight cycles. The effects of weight cycling on rates of weight gain and loss, caloric efficiency, and ad lib wheel running were investigated with three groups of adult female rats: 1) cycling (cycled twice); 2) maturity control (cycled once); and 3) chow control (not cycled). The cycled group evidenced weight-gain periods of 36 and 21 days, respectively, and showed a significant increase in food efficiency during the second weight-gain period, relative to the first. There was no evidence that maturation was responsible for this phenomenon. Time required to lose weight and ad lib wheel running were not influenced by weight cycling. These findings suggest that weight cycling may make maintenance of normal weight more difficult and have implications for human weight-control programs.