Abstract: Objective To measure the intracellular free Ca²⁺ (Ca²⁺_i)by the study of the Ca²⁺ sensitivity of the calciumactivated potassium channel(KCa)in hypothalamic neur ons of new- born SD rats through patchclamp technique.Methods Firstly,by using insid-eout configuration the channel open probabilities(P₀) was aqcuired when different(Ca²⁺_i)were adopted,then the concentration-effect curve was worked out.In cell attached configuation,we obtained the P₀ of the KCa on hypothalamic neurons in physiological condition,then checked out the(Ca²⁺_i)corresponding to the P₀ on the concentration effect curve.Results KCa had a high sensitivity to(Ca²⁺_i)and the(Ca²⁺_i)of hypothalamic neurons was 0.1μmol/L.Conclusion The electric-physiological method had high accuracy and reliability,it provided a extensive prospects for the study of cell signal transduction