126 micropunctures were performed on 40 Bowman's capsules as well as along the corresponding proximal convoluted tubules (PCT) on six nondiuretic rats. Capsular and tubular fluid samples (GF and TF) collected were analysed for Na, Cl, K, P, Ca Mg and 3H-Inulin concentrations by electron probe analysis and liquid scintillation. Electrolyte handling along the usually inaccessible part of the PCT, i.e. EPCT, was determined by selecting from the samples collected in the first two or three proximal loops, those having a (TF/GF) In value smaller than or equals to 1.15. Except for Na the concentration of electrolytes was significantly modified when the fluid flowed along the PCT: 1. In the EPCT, (TF/GF)Cl and K rose and (TF/GF)P dropped in correlation with (TF/GF)In. For (TF/GF)In = 1.15, (TF/GF) chloride, potassium and phosphate were respectively equal to 1.12, 1.10 and 0.72. These values were comparable to thoose obtained in the remaining length of PCT, indicating that the chemical gradient established in the first loops was maintained in the subsequent ones. Therefore proximal handling of these three ions differed depending on the PCT portion considered. The reasons for this difference are discussed further on. 2. Calcium concentration increased and reached a plateau corresponding to (TF/GF)Ca = 1.22 for (TF/GF)In values ranging from 1.31 to 1.45. 3. Magnesium was not reabsorbed for (TF/GF)In values below 1.90. Above this level slight Mg reabsorption occurred. 4. Significant correlations between (TF/GF)K, Ca and P and (TF/GF)Cl ratios were observed along the PCT, suggesting that proximal reabsorption of K, Ca and P was dependent on tubular fluid acidification.