Atrial myocardium of guinea pigs was used to study effects of Escherichia coli endotoxin shock on inotropic characteristics of heart muscle free from noncardiac influences of the in vivo shock state. In vitro exposure of atrial muscle to large concentrations of endotoxin (10-1,000 microgram/ml, final concentration) for a prolonged period (90 minutes) had no effect on myocardial contractility. However, atrial muscle isolated from endotoxin-shocked guinea pigs exhibited clear evidence of mechanical depression, as reflected by markedly low values for both isometric contractile tension and maximal rate of tension development (dT/dt). Also, since systolic and diastolic time intervals of myocardial contractions were not discernibly affected by shock, the contractile deficit represented a true inotropic dys-function and was not due simply to a temporal change in the active state of the muscle. The shock-induced inotropic disorder was permanent enough to persist in vitro for several hours of observation. However, if the Ca++ concentration of the bathing medium was increased from 2.5 mM to maximally effective concentrations ( greater than 4.5 mM), contractile strength of heart muscle from the shocked group was equal to corresponding responses of control muscles. Present findings verify myocardial contractile dysfunction associated with in vivo endotoxin administration and provide characterization of a test system that should prove useful for further study of functional changes occurring to the heart in shock.