Bcl-2-associated athanogene 3 (BAG3) is strongly expressed in cardiac muscle. To understand the functional role of cardiac BAG3, we generated TG mice that overexpress BAG3. A decrease in fractional shortening, and the induction of cardiac ANP, were observed in BAG3 TG mice. Moreover, a marked reduction in the protein level of small HSPs was detected in BAG3 TG mouse hearts. The protein turnovers of small HSPs by the autophagy system were activated in BAG3 TG mouse hearts. Thus, BAG3 is critical for the protein turnover of small HSPs via activation of autophagy in the heart. In order to address the relationship between the reduced cardiac function as well as the induction of stress markers and autophagy activation in BAG3 TG mouse hearts, the effects of chloroquine treatment on cardiac function as well as cardiac ANP level in BAG3 TG mice were examined. Twenty hours after chloroquine treatment, reduced fractional shortening was recovered to a normal level in BAG3 TG mice concomitant with marked attenuation in induced cardiac ANP. These results suggest that inhibition of the activated autophagy in BAG3 TG mouse hearts can rescue the phenotypes by BAG3 overexpression in the hearts.