The rat pericoronary adipose tissue was perfused in the presence of either the liposynthetic hormone insulin or the lipolytic hormone noradrenaline. Insulin perfusion associated with a) larger adipocyte mean sectional diameter in comparison with noradrenaline perfusion; b) glycogen deposition; c) appearance of small fat globules at discrete sites at the periphery of the main lipid drop. The two latter phenomena were apparently dose-dependent. Massive lipid deposition was induced by addition of triglycerides to the perfusion medium and this associated with appearance of prominent endoplasmic reticulum in the cytoplasm. In noradrenaline-perfused adipose tissue many small lipid droplets surrounded the central lipid deposit and the endoplasmic reticulum was in the form of both thin long, dashed cisternae sometime surrounding lipid droplets and grouped, anastomosing tubular cisternae. The present work shows that the perfused white adipose tissue of the heart is a suitable model to study, in situ, the morphological effects of hormones in adipocytes.