During the early larval life of lampreys, connective tissue strands develop between the notochord and oesophagus, thereby suspending the oesophagus between the two ducts of Cuvier and above the heart, which, unlike adult lampreys, is not enclosed in a rigid pericardial capsule. Examination of histological sections from proammocoetes and young ammocoetes provides strong evidence that the growth of this suspensory tissue is responsible for the dorsal aorta moving to the right and undergoing a concomitant rotation, which shifts the origin of the anterior mesenteric artery on the dorsal aorta from a right ventro-lateral to a right dorsolateral position. While the heart initially develops the three major layers found in craniates, i.e. epicardium, myocardium and endocardium, the innermost of these breaks down early in larval life. The myocardium thus becomes bathed in blood, a feature which would help compensate for the lack of a coronary artery. After studying the way in which they are formed early in life, it is proposed that the epicardium is best termed the visceral or splanchnic pericardium and that the thin membrane lining the cavity surrounding the heart should be referred to as the parietal or somitic pericardium. The pericardial capsule of the adult, which is made of non-compliant walls of cartilage and connective tissue. consists of three main components. The anterior component comprises the anteriorly convex post-branchial wall. The intermediate component is formed by the posterior part of the seventh branchial arch on each side of the body and their posteriorly-directed processes. The latter processes expand into the cup-shaped pericardial cartilage, which constitutes the third and most posterior component of the pericardial capsule. A pear-shaped opening in the connective tissue formed between the two anteriorly-directed dorsal extensions of the pericardial cartilage allows the cardinal veins to join the single (right) duct of Cuvier, which in turn enters the sinus venosus. The ventral part of the cartilaginous floor of the pericardial capsule contains a large slit of connective tissue through which the ventral hepatic vein passes upwards to enter the sinus venosus. The inner surface of the pericardial capsule contains an elaborate and very well developed network of venous sinuses and sinusoids, which are supplied by blood from the peripharyngeal sinus and are drained by the inferior jugular vein. It is suggested that this network produces fluid which helps lubricate the heart. A description is given of the way in which the adult pericardial capsule is developed during the seven stages of metamorphosis. This sequence demonstrates that much of the development of the capsule takes place during stage 3 and that by the end of stage 6 the form of the capsule has become essentially the same as that of young adults.