Scanning electron microscopy of vascular casts of the pancreas from monkies, cattle, pigs, dogs, cats, rabbits, guinea pigs and mice showed certain species differences in the occurrence of intralobular and interlobular islets and in the microcirculatory pattern of these islets. Interlobularly located islets were frequently found in the mouse and guinea pig, as has been previously established in the rat (Murakami and Fujita, 1992); they emitted insulo-venous efferent vessels directly draining into veins. In contrast, the intralobular islets in the guinea pig usually issued insulo-acinar portal vessels continuous with the lobular capillary network. In the mouse, they usually emitted both the insulo-acinar portal and insulo-venous efferent vessels. The insulo-venous efferent vessels, including those of the interlobular islets, could partly be portal in nature since they occasionally issued portal branches directed to the lobular capillary network. In rabbits, cats, dogs, pigs, cattle and monkies, as in men (Murakami et al., 1992), essentially all islets in the pancreas were intralobular in location and usually emitted the portal vessels only. In the mouse and rabbit, as in the rat (Murakami and Fujita, 1992), the islet received afferent vessels in its superficial aspect and issued efferent vessels from its deep aspect. In the Formosan monkey, as previously reported in the rhesus monkey (Fujita and murakami, 1973), the afferent vessels usually ran deep into the islet which emitted vessels from its superficial aspect. In other animals examined in this study, as in humans (Murakami et al., 1992), no consistent rule concerning the microcirculatory pattern within the islet could be determined.