The influence of propranolol and verapamil, i.e. two cardiodepressant drugs differing in their pharmacological actions, on cellular autophagy in the left ventricular myocardium of the rat was investigated. In the first experimental series 10 animals were given propranolol subcutaneously (3 mg/kg body weight). Ten controls received physiological saline. In the second series 8 animals were treated with verapamil subcutaneously (9 mg/kg body weight) and 8 controls with physiological saline. Two to 4 h after the injections in the first series and 1 3/4 to 3 h in the second series retrograde perfusion fixation was carried out via the abdominal aorta with a paraformaldehyde-glutaraldehyde mixture. Myocardial tissue from the left anterior wall was processed for electron microscopy and was morphometrically evaluated for volume fraction and numerical density of early stages of autophagic vacuoles (AVs). Propranolol and verapamil significantly increased the AV volume fraction 4.3- and 2.7-fold, respectively. The numerical density was increased by the two drugs, although to a lesser degree (2.3-fold, and 1.7-fold, respectively). As verapamil affects neither the beta-adrenoreceptors nor the intracellular levels of the second messenger cAMP, the only common denominator for the stimulation of cellular autophagy seems to be the cardiodepressive effect of the two drugs. The data suggest that the rise in cellular autophagy is an early regulatory step in the adaptation of heart muscle mass to reduced work load.