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4. The mechanisms underlying the apparent decrease in the sensitivity of the contractile proteins to Ca2+ were investigated by applying rapid length changes to papillary muscles at control pH., during acidosis, and after bathing [Ca2+] had been increased to match force during acidosis to that in control.5. Acidosis decreased the change in force produced in response to a given length change (i.e. decreased muscle stiffness) but when bathing [Ca2+] was increased during acidosis, muscle stiffness returned to control.6. Acidosis had no effect on muscle stiffness after the induction of rigor in the muscle (produced by metabolic inhibition).7. It is suggested that in intact cardiac muscle the major effect of a mild acidosis is to decrease the sensitivity of the contractile proteins to Ca2+, hence decreasing the number of bound cross-bridges.