Light and electron microscopical studies on pig hearts have confirmed the presence in the right and the left atrial endocardium of distinct circumscribed, unencapsulated end-organs (baroreceptors) associated with 4-9 mum thick nerve fibres. The myelin sheath terminates before the entrance of the nerve into the end-organ. The regular presence of thin axon profiles containing clusters of small agranular vesicles, both around the thick nerve fibre and in the periphery of the end-organs, suggests a double innervation. The thick fibre arborizes inside an aggregate of Schwann-like cells to form a large number of terminals which are considered to be the mechano-sensitive receptors of the organ. In addition to the nervous structures and the Schwann-like cells the organs contain a varying number of connective tissue fibrils. The terminals are covered by a basement membrane but are partly or completely devoid of Schwann cell covering. Most of the space inside the terminals is occupied by tightly packed mitochondria. Glycogen granules are regularly present, sometimes abundant. SER occurs widely, frequently forming complexes with glycogen granules. These complexes may be highly organized as parallel or concentric structures, suggesting a close connection between SER and glycogen metabolism in the terminals. The terminals are regularly furnished with small protrusions containing numerous 30-100 nm clear vesicles which resemble pinocytotic vesicles. In addition, a few 80 nm dense-cored vesicles are found. The occurrence of numerous cyto-segresomes and lipofuscin-like bodies suggests a lively turnover of organelles in the terminals.