The cineangiographically determined mean velocity of circumferential fiber shortening (-vcf) has been used as a measure of basal cardiac contractility in man. In order to test the effect of acute changes in preload on -vcf, dextran was administered i.v. in steps of plus 2 percent, plus 4 percent and plus 6 percent of body weight to 8 closed-chest anesthetized dogs after minimizing cardiac reflex adjustments by 0.5 mg/kg propranolol i.v. and by cutting the vagi. -vcf, end-diastolic and end-systolic volume were estimated from left ventricular (LV) monoplane cinefilms in the right anterior oblique projection. At plus 2 percent and plus 4 percent of dextran -vcf remained unchanged as compared to control, i.e. after autonomic blockade. At plus 6 percent -vcf decreased significantly. LV max dP/dt increased significantly up to the step of plus 4 percent and decreased then slightly despite a further significant increase of LV end-diastolic pressure. In additional 6 dogs the largest volume load (plus 6 percent) was applied as the first step after control. Under these conditions there was no significant change in -vcf. In conclusion, -vcf appears to be essentially independent of changes in preload. Its usefulness as a measure of contractility is however limited by the negative inotropic effect of repeated injections of contrast dye.