Changes in blood volume can induce morphometric and morphological alterations in the secretory complex of the myoendocrine cells due to the stretching of atrial walls. These alterations were studied by electron microscopy, using dogs infected intraperitonially with Trypanosoma cruzi and necropsied during the acute phase of the infection when congestive heart failure was present. Several changes were observed in the myoendocrine cells of the heart: hypertrophy and hyperplasia of rough endoplasmic reticulum and Golgi complex, increase in telenuclear secretory complex, increase in fusion of type B atrial specific granules (ASG), decrease of the total number of ASG, enlargement of the maximum diameter of type A ASG and a relative increase in the number of type B ASG. These alterations suggest a larger secretory activity of the atrial myoendocrine cells with a larger secretion of atrial natriuretic peptide (ANP).