The classical force-velocity relationship is a standard measure of the contractile function of isolated linear cardiac muscle, but no such simple index of contractile function exists for the isolated mammalian cardiocyte. Therefore, this study established an analogous viscosity-velocity relationship for the characterization of cardiocyte contractile function. For this purpose, force was imposed on unfettered adult feline cardiocytes as a series of defined viscous loads, which provided resistance to cardiocyte shape changes during contraction. This was done by increasing the viscosity of the Krebs superfusate (37 degrees C, pH 7.4) in graded, reproducible steps from 1 to 500 centipoise by the addition of methylcellulose. Sarcomere motion within each contracting cardiocyte was measured as movement of the diffraction pattern cast onto a photodiode array by a laser beam passing through the cell. Both the rate and extent of sarcomere shortening varied inversely with increasing viscosity, whereas neither resting sarcomere length nor osmolarity was altered. Further, increased inotropism effected by paired-pulse stimulation of cardiocytes caused an upward shift of the entire viscosity-velocity relationship. Thus the cardiocyte viscosity-velocity relationship is analogous in form to the force-velocity relationship of isolated linear cardiac muscle and provides a simple reproducible method for characterizing the contractile performance of relatively large numbers of cardiocytes isolated from a single specimen of myocardium.