The ultrastructure of myocardial cell nuclei was examined in a group of 12 patients undergoing cardiopulmonary bypass surgery. In one patient (aged 60 years) with rheumatic heart disease, myocardial cells were observed which contained mitochondria with well preserved membranes within their nuclei. Such intranuclear mitochondria were observed with equal frequency in all of the three stages examined, i.e. at the start and end of aortic crossclamping and after 20 min of reperfusion. Approximately 2-3% of the total number of nuclei studied in this patient contained mitochondria. A partial disintegration of the membranes of the affected nuclei was sometimes seen. These findings are discussed in relation to the hypothesis of a de novo synthesis of mitochondria inside the nucleus, and also, in relation to the "trapping theory" of mitochondria through the nuclear envelope.