Auxotonic work/beat and contraction curves were recorded from left guinea pig atria stimulated with 0.5 Hz at 5 mN preload in K.H.-solution + 15 mmol/l glucose, 95% O2 or N2 + 5% CO2, 30 degrees C. Eight common parameters of the contraction curve were calculated simultaneously in a computerized system as reported previously (Englert, R. et al., N.-Arch. of Pharmacol., 337, Suppl. R. 60, 1988). By increased anaerobic glycolysis anoxia reduced contractile activity which after 30 min reached a plateau of approximately 50% of the initial aerobic value during approximately 2 hours. At reoxygenation the contraction force increased with a first peak overshooting 50% of the initial aerobic value after 5-10 min, to decline during the following 10-15 min to a plateau slightly below the initial aerobic value. Several parameters exhibited characteristic changes during anoxia and reoxygenation: during the first minutes of reoxygenation in the ascension of the first peak the 'time to peak force', the 'relaxation time' and the 'area under the contraction curve', especially the part below the relaxation, were strongly but only transiently increased. Positive and negative inotropic agents influencing cytosolic free Ca2+ in different ways applied 15 minutes before reoxygenation indicated that the observed overshooting peak of force and the prolongation of contracation duration after reoxygenation were presumably due to transiently disturbed intracellular Ca2+ redistribution in sarcoplasmic reticulum and mitochondria during the restitution of high energy phosphates.