One of the most prominent nanoproducts is the silver nanoparticle (AgN), since silver ions exhibit remarkably unusual physical, chemical and biological properties. Creatine kinase (CK) plays a central role in metabolism of high-energy consuming tissues such as brain, skeletal muscle and heart, where it functions as an effective buffering system of cellular ATP levels. A decrease of CK activity may impair energy homeostasis, contributing to cell death. In the present work we studied the in vitro effect of AgN on the activity of CK from rat brain, heart and skeletal muscle. Our results demonstrated that AgN (10, 25 and 50 mg L-1) in vitro inhibited CK from brain and skeletal muscle, but not from heart. Several works showed that AgN cytotoxic effects probably involve interaction between silver ions and sulphydryl groups of proteins. We suggest that AgN inhibited CK activity through interactions with thiol groups of the enzyme.