The morphology of the microcirculation of the pancreas in 20 monkeys (Macaca mulatta) was studied by scanning electron microscopy (SEM) of vascular corrosion casts and light microscopy (LM) of China ink-injected/cleared tissues. The principal results were that 1) insulo-acinar portal vessels were found between the endocrine and exocrine parts in the pancreas. The blood flows from the endocrine to the exocrine part. 2) Depending on the different microvascular arrangement, there were two patterns of microcirculation in the islet: in 66% of islets the direction of microcirculation was from cortex to core, and in 44% from core to cortex. 3) Islets could be categorized in three classes on the basis of size: the small islets (40-100 microns in diameter), the intermediate islets (101-240 microns in diameter), and the large islets (241-340 microns in diameter). 4) Insulo-insular portal routes were observed in the pancreas of the monkey. Some intermediate or large islets were connected to an adjacent small islet by one or two, occasionally more, efferent vessels. These small islets received no arterial branch and were entirely supplied by the portal vessels--the efferent vessels of intermediate or large islets. The authors suggest this new pattern to be termed the insulo-insular portal system. 5) A single centrally located intralobular artery as the exclusive vessel supplied each pancreatic lobule of the monkey, there being no anastomosis between the intralobular arteries and any of their branches. This anatomic feature might be the morphological basis of the pancreatic microcirculatory disturbance and microvascular impairment occurring during acute pancreatitis.(ABSTRACT TRUNCATED AT 250 WORDS)