The contractile effects of 1.1 mM amrinone were studied on isolated skeletal muscle fibres of the frog (2-5 degrees C, slack length). Amrinone potentiated the twitch amplitude and the maximum tetanic tension and also enhanced the maximum contracture response elicited by 117.5 mM K+ (mean increase 6.3 +/- 1.8%, n = 6, P less than 0.02). The time to half-relaxation of the potassium contracture was slightly increased (mean change 13.8 +/- 3.5%, n = 7, P less than 0.01). Amrinone furthermore shifted the S-shaped curve relating contracture tension to log K+ to the left, and thus reduced the threshold depolarization (the mechanical threshold) needed to elicit a contracture (mean reduction 11 +/- 1 mV). The duration of the action potential at the -25 mV level was slightly increased by amrinone, whereas the resting membrane potential was unaffected.