Electron Probe Analysis (EPA), using a MS 46 microprobe (Cameca, France), has been developped and applied to the determination of Cl, Na, K, Ca, P and Fe concentrations in 48 micropuncture samples collected from proximal and distal convolutions in three rats perfused with both ferrocyanide and tritiated inulin, and undergoing salt diuresis of various intensities. The technique for EPA, as described, uses 0.4 ml samples of tubular fluid (TF), plasma ultrafiltrate (Pf) and standard solutions, all treated in the same way. The reliability of EPA for micropuncture samples was demonstrated by the following control observations: ferrocyanide (Iron EPA) and 3H-Inulin determinations lead to comparable and highly correlated (r = 0.988) TF/P; simultaneous determinations of Na and K concentrations by both EPA and ultramicroflame photometry resulted in regression lines, the slope of which was very close or equal to 1. Moreover, concentration values found by EPA for all the elements studied in plasma ultrafiltrate were in good agreement with those obtained by other methods. From the EPA data, tubular handling for Cl- Na+, K+, Ca++ and phosphate ions could be observed; the results generally confirm those previously reported by others. A few more original informations, obtained for Mg++ in the last experiment are described with some details. It is concluded that Electron Probe Analysis is a highly suitable method in micropuncture experiments.