The ultrastructural effects of pressure overload hypertrophy and its subsequent regression were studied in three regions of the left ventricular myocardium of ferrets. Quantitative analysis of electron micrographs was carried out by computer analysis. Four animals were studied 14 days after a 70% constriction of the ascending aorta. Five animals were similarly constricted for 14 days but were studied 6 weeks after the aortic band had been removed. Five animals served as controls. 70% aortic constriction produced a 45% increase in LV mass. Ultrastructural examination showed no qualitative difference between hypertrophy and control groups. Quantitative analysis showed an increased mass of mitochondria and myofibrils, but the fractional cell volume of mitochondria and myofibrils was unchanged in hypertrophy as was the mitochondria/myofibril ratio. In the hypertrophy group, regional differences in fractional cell volume of myofibrils were noted. After 6 weeks of debanding, left ventricular mass was not different from controls. No qualitative ultrastructural changes were found in the regression group, and fractional cell volume of mitochondria and myofibrils showed no difference between regression and controls. However, regional differences were again noted in the fractional cell volume of myofibrils for regression animals. The mitochondria / myofibril ratio of regression and control groups was not different, nor was the mitochondrial and myofibrillar mass different. For this hypertrophy model normal ultrastructure is maintained in hypertrophy and regression. Quantitative morphometry suggests that a normal balance between mitochondria and myofibrils is maintained in the myocardium after 14 days of hypertrophy and 6 weeks of recovery from hypertrophy.