The ventricle morphology is described in hearts in which the entire atrial inputs are committed to a single chamber in the ventricular mass, this being considered the criterion for definition of the univentricular heart. Such a definition does not exclude from the univentricular category hearts with two chambers in their ventricular mass providing that the second chamber does not receive a direct atrial input. Three types of ventricular morphology were identified: hearts with a main chamber of left ventricular pattern and a rudimentary chamber of right ventricular pattern; hearts with a main chamber of right ventricular pattern and a rudimentary chamber of left ventricular pattern and hearts with a sole chamber in their ventricular mass of indeterminate pattern. Hearts may also exist of right or left ventricular type without rudimentary chambers. Variation was noted in type and position of the rudimentary chambers. Chambers of either right or left ventricular type were found supporting either or both great arteries (outlet chambers), and also without a direct outlet (trabecular pouches). Right ventricular rudimentary chambers were usually anterior but could be right or left sided; left ventricular chambers were posterior but again could be right or left sided. Hearts with these ventricular morphologies possessed either double inlet atrioventricular connexions, or absence of the right or left atrioventriuclar connexion. In double inlet ventricles, cases were observed with two valves, a common valve, one imperforate valve, and with straddling atrioventricular valves. A straddling valve was also found in hearts with absence of one atrioventricular connexion. Further variation was found in the pattern of arterial connexions. It is concluded that ventricular morphology is an important feature of the iniventricular heart but must be considered in the context of other important and variable features.