The effects of verapamil (5 mg/l) and caffeine (1 g/l) on two-component contraction were studied in isolated papillary muscles of guinea pigs, bathed in normal Tyrode's solution at 20-22 degrees C, containing 2 mg/l noradrenaline. The 1st derivative of contractile response and transmembrane AP were simultaneously recorded. Verapamil suppressed the amplitude of the 2nd component and did not affect the 1st component of contraction. Caffeine eliminated the 1st component and somewhat increased the 2nd component of contraction. The Ca ions from two different pools seem to take part in activation of the two-component contraction: 1) released from sarcoplasmic reticulum and 2) crossing a cell membrane during the plateau of the AP.