It has previously been established that MAO-A activity markedly increases (about 600%) with age in the rat heart. The aim of the present study was to examine the effect of age on the activity of MAO-A and -B in the mouse heart. In contrast to the rat heart, beta-phenylethylamine is deaminated by MAO-B in the mouse heart. Heart MAO-B activity was found to significantly increase (about 70%) with age in both male and female mice, whereas MAO-A activity remained unchanged. Compared to age-matched rats, serotonin-deaminating activity was about 35 times and 204 times lower in young and old mice, respectively.