The length-dependence of force-frequency relations in rat heart was studied in isometrically contracting papillary muscles. Negative force-frequency relations were observed for all lengths; the slope of the optimal length curve (Lopt) corresponding to peak contractile force during rhythmic pacing at 0.5 Hz, was greater than that for both 0.7 Lopt and 1.20 Lopt. The slope of static force-frequency curve was estimated at various muscle lengths in the range between 70 and 120% of Lopt. It was concluded that: 1) the heart muscle length corresponding to the maximal peak contractile force depends on stimulation frequency; 2) the slope of the force-frequency curve is maximal at the muscle length of 90 to 95% optimal length determined at basis stimulation frequency (0.5 Hz). The operating point for heart muscle in vivo is believed to be biased from the maximum of length-tension curve to its ascending part.