There is described a method of magnetically manipulating a cell in vivo which comprises the association of a magnetizable particle with a cell. More particularly, there is described a method of magnetically manipulating a cell which comprises the association of a magnetizable particle with a cell characterized in that the method comprises agonizing or antagonizing ion channels within a cell by the association of a magnetizable particle with a cell. There is also described the use of a magnetizable particle in a method of magnetically manipulating a cell in vivo and/or activating ion channels in vivo.