Cardiac actomyosin ATPase was increased by making rats swim 150 min/day, 5 days/wk for 8 wk. Changes in Ca2+ -stimulated ATPase activity were then studied in these conditioned rats and in similarly conditioned animals in which swimming was subsequently discontinued (group A), reduced to 45 min/day (group B), or continued at the original level for an additional 8 wk (group C). After the 8-wk initial program actomyosin ATPase activity averaged 22% higher in hearts of conditioned rats than in hearts of sedentary controls (P is less than 0.001). In group A, actomyosin ATPase activity declined rapidly and reached the level found in sedentary controls by the 13th day. In group B, actomyosin ATPase activity declined to the control level by the 30th day. At the end of 16 wk the percent increase in actomyosin ATPase activity in group C over that in hearts of sedentary animals was approximately the same as after 8 wk. These results demonstrate that elevation in cardiac actomyosin ATPase caused by moderate physical training in rats is not maintained if the training program is decreased or discontinued. The training program must be continued at or near the initial level if the increases in cardiac actomyosin ATPase are to be sustained.