Isometric tetani of single muscle fibers of Rana temporaria were studied as a function of stimulation rate, sarcomere length (1.7-2.3 micron), twitch-to-tetanus ratio, and exposure to twitch potentiators (Zn2+ and NO3-) at 20 degrees C. As the stimulation rate was decreased below a maximal level, tension generation decreased. This depression in tension generation was more pronounced at shorter sarcomere lengths. Therefore the magnitude and shape of the sarcomere length curve was dependent on stimulation rate. Although the depression in tension generation was always accompanied by a noticeable ripple in the tension record in fibers with large twitch-to-tetanus ratios, it could be observed even during well-fused tetani in fibers with low twitch-to-tetanus ratios. In all fibers, however, high stimulation rates or exposure to potentiators resulted in maximum tension generation at each length, and the sarcomere length-tension curve followed that found by Gordon, Huxley, and Julian. This indicates that the fall in tension between sarcomere lengths of 2.0 and 1.7 micron is not due to length-dependent activation but is more likely to be the result of mechanical interference in the force-generating interaction between cross bridges and thin filament sites.