A method of sensing the concentration level of at least one particular electrolyte in the sweat fluid of a subject, includes providing a sweat sensor system having sweat fluid absorbing material, measuring apparatus for sensing the electrical conductivity of sweat fluid absorbed by the absorbing material and producing ionic concentration data for said at least one particular electrolyte, and a user interface connected to the measurement apparatus for interpreting the data to a user. The absorbing material is applied to the skin of the subject to cause sweat fluid thereon to be absorbed by the absorbing material whereby the measurement apparatus produces ionic concentration data for the sweat fluid absorbed and the user interface interprets the data for a user.