In a study of the aortic component of the second heart sound (A2) intensity. using 10 dogs, miniature cardiac accelerometers were attached to the exposed left ventricle (LV) at the apex, and to the surface of proximal aorta, to obtain LV and aortic surface acceleration. Manometer tipped catheters were used to detect aortic systolic pressure and diastolic pressure and time constant of left ventrucular pressure fall "T". Drugs (Nitroprusside. Dobutamine and Methox-amine) altered aortic pressure, LV contraction and LV relaxation. When T was ±35% of control, the aortic systolic pressure and diastolic pressure were good predictors of A2 intensity. When LV relaxation was impaired, increasing T>135% of control, the A2 intensity for any given aortic pressure was reduced. When relaxation was hyperactive, decreasing T<65% of control, A2 intensity was increased. Aortic pressure/T which assessed both aortic pressure and relaxation ability. is a better determinant of A2 intensity than aortic systolic pressure or aortic diastolic pressure alone.