The sodium-calcium exchange activity has been studied in sarcolemmal vesicles isolated from rat ventricles hypertrophied by pressure overload. 4 weeks after aortic stenosis the degree of hypertrophy varied from 30 to 70%. The Na+-dependent 45Ca2+ influx and efflux were up to 50% decreased and the sensitivity to Ca2+ was 13-fold lower in vesicles from hypertrophied heart as compared to those from normal heart. However, the Na+,K+-ATPase activity, the orientation of the vesicles and the passive Ca2+ permeability were found to be similar in the two heart groups. These results indicate that the sarcolemmal Na+/Ca2+ exchange activity could be qualitatively and/or quantitatively changed in hypertrophied rat heart.