The passive-mechanical and dynamic properties of the rabbit inferior oblique muscle IO were studied in vitro at 35 degrees C. The influence of length on the resting tension and isometric contractions were determined. Maximum twitch tension and fusion tension were developed at optimum length (Lo) an extension of the muscle to about 1.15 times LR, the resting length of the IO in situ. A linear relation was found between length and tension in the activated muscle. An increase in stimulation frequency induced a parallel shift in the curves to higher tension but the slope of the curves remained unchanged. On an average the IO had in response to direct massive stimulation a twitch contraction time of 6.4 ms and a half-relaxation time of 7.0 ms. At stimulation with 300 Hz or above the tetanus fused. Stimulus frequencies above fusion frequency increased the rate of tension rise but not the maximum tetanic tension. The maximum tetanic tension was about 6.4 N/cm2, and the twitch:tetanus ratio was 0.1. To prolonged tetanic stimulations the IO exhibited a high fatigue resistance. Cooling the muscle to 25 degrees C was followed by an increase in the time parameters of single twitches and tetanic contractions, a decrease of the tension developed in a fused tetanus and a small potentiation of the twitch. Following a repetitive stimulation a small post-tetanic potentiation of the twitch was observed.