Experiments were designed to 1) study in situ changes in papillary muscle length and force during the cardiac cycle, 2) investigate the relationship between papillary muscle length and maximal ventricular pressure, 3) study the effect of both positive and negative inotropic intervention on this relationship. A mercury gauge transduceror Walton-Brodie strain-gauge arch was sutured to the anterior papillary muscleand used to measure the extent of shortening or lengthening (deltaL) or force (APMF),.respectively. The anterior papillary muscle showed rapid increase of length and forcewhile contracting during isovolumic contraction and reached peak systolic length at end-isovolumic contraction or during the early injection phase. The papillary muscle was observed to shorten during the phases of ejection and isovolumic relaxation. It was concluded that changes in ventricular pressure during the cardiac cycle are associated with changes in papillary muscle length and force such that the resulting "lengthening contraction force" is appropiate for maintenance of normal atrioventricular valve function during isovolumic contraction. There was an inverse relationship between deltaL and maximal ventricular pressure such that for each increment in peak ventricular pressure there was a decrease in deltaL. The deltaL-force relationship was shifted upward and to the right by norepinephrine (NE) and isoproterenol and downward and to the left by occlusion of the posterior vena cava. Phenylephrine had little or no effect on this relationship.