A luminal microperfusion technique was used to examine the K⁺ permeability of surface proximal convoluted tubules (PCT) in the kidney of anesthetized rats. Transtubular potassium concentration ([K⁺]) gradients were varied by altering the concentration of KCl in luminal perfusates, to which 32 mmol/l of the impermeant solute raffinose was also added to prevent net fluid reabsorption. The arithmetic mean transtubular [K⁺] gradient was highly predictive of net potassium flux, yielding an apparent K⁺ permeability of 31.9 ± 1.7 × 10⁵ cm/s in the absence of fluid reabsorption. When compared using identical calculation techniques, we found this was not significantly different from the permeability derived in a previous study when fluid reabsorption was present [J. D. Kibble, M. Wareing, R. W. Wilson, and R. Green. Am. J. Physiol. 268 (Renal Fluid Electrolyte Physiol. 27): F778-F783, 1995]. We conclude that fluid reabsorption does not affect the apparent permeability of the proximal tubule to potassium. The apparent permeability to ⁸⁶Rb, measured following its addition to luminal perfusates, was not significantly different from the value obtained for K⁺, suggesting that rubidium is a useful marker for net potassium movements in the PCT of the rat.