This study was designed to assess whether maximum shortening velocity and force-velocity parameters were reduced by acidosis in rat skinned cardiac trabeculae, as reported for skinned skeletal muscle. Thin trabeculae were dissected from right ventricle of adult male Wistar rats, chemically skinned, and maximally Ca-activated at pH 7.0, 6.6 and 6.2. Maximum tension (Po), maximum shortening velocity evaluated from force-velocity curves (Vmax), and maximum power output (Wmax) were significantly reduced for exposure to pH 6.6 and 6.2. When the value at pH 7.0 is taken as reference, Po decreased by 30% and 49%, Vmax by 17% and 40% and Wmax by 30% and 60% at pH 6.6 and 6.2 respectively. A similar reduction (25% at pH 6.6 and 42% and pH 6.2) was observed when unloaded shortening velocity (Vo) was determined with the slack test method. The results obtained show that both Po and Vmax or Vo are affected by a substantial decrease in pH. The combination between significantly lower Po and Vmax gives rise to significantly lower Wmax. The reduction of maximum shortening velocity (Vmax or Vo) can be explained by assuming either the existence of an internal load or an impairment in actin-myosin interaction.