1. Suction blisters on the skin of rats, obtained with a negative pressure of 150 mm Hg, show a close similarity to human suction blisters, based on histological, electrolyte and protein findings. 2. Pharmacokinetic experiments with inulin and antipyrine have proved that the movement of these low protein binding drugs between blister fluid and serum follows the laws of diffusion. 3. The investigations suggest that blister fluid resembles interstitial fluid and can therefore serve as a model for the estimation of drug concentrations in interstitial fluid.