To observe the changes in myocardial function and calcium ion release of sarcoplasmic reticulum in rats under simulated weightlessness, 24 Sprague-Dawley male rats were divided into control (CON), 8 week tail-suspension (SUS) and recovery for 2 weeks (RE) groups. The results showed that there was no change in resting tension of myocardial twitch in SUS. While the developed tension (DT) of myocardial twitch significantly decreased in SUS as compared with CON. The time to peak tension (TPT) and time to half relaxation (T1/2R) of myocardial twitch were prolonged in SUS. DT, TPT and T1/2R recovered to their corresponding control values after 2 w of recovery from tail-suspension. The early recovery curve of force-interval relation in SUS was in a higher position over that of CON, but the rest potentiation and rest depression curve was positioned under that of CON. These results suggested that there might be an adaptative change in myocardial function in medium- or long-term tail-suspended rats while 8 w tail-suspension reduces calcium ion release from cardiac sarcoplasmic reticulum in rats.