The fine structure of the perfused spleen of the rainbow trout (Oncorhynchus mykiss) was studied by scanning electron microscope. Morphological comparisons were carried out between the spleen in dilated (resting) condition and in contracted condition induced by 5.5μM adrenaline infusion. Sphincter-like constrictions of the arterioles terminate additional blood supply to the spleen during splenic contraction. It is suggested that reticulum cells contributed to the marked decrease in splenic volume and expulsion of blood cells stored in the reticular meshwork following adrenergic stimulation. The fenestrae found in the walls of the proximal section of the venules function as major passages enabling erythrocytes to be propelled from the reticular meshwork to the vascular lumen.