The cytoplasm of the rat isolated fat cell contains a highly organized, interconnected system of smooth endoplasmic reticulum having close association with the central lipid mass, mitochondria and cytoplasmic lipid droplets. Elements of smooth endoplasmic reticulum approach, but do not fuse with, the plasma membrane. When fat cells were treated with potassium pyroantimonate during fixation for electron microscopy a precipitate was produced inside the smooth endoplasmic reticulum. Analysis with an AEI-EMMA 4 analytical electron microscope showed that the precipitate contained calcium but not sodium, magnesium or manganese. It is possible that the smooth endoplasmic reticulum of fat cells may be structurally and functionally analogous to the sarcoplasmic reticulum in skeletal muscle, and that redistribution of calcium from a calcium store inside the smooth endoplasmic reticulum may be a consequence of the action of lipolytic hormones.