(1) Fluorescent compounds and specific ion currents (Na+, K+) were used to study the kinetics of the diffusional exchange between small cells and patch pipettes in the tight seal whole cell configuration of the patch clamp technique. (2) Changes in the intracellular concentration of the test substances following patch rupture could be fitted with single exponentials, provided the access resistance RA of the pipette remained constant during diffusional equilibration. The diffusion time constants were linearly related to the access resistance. (3) When apparent diffusion rates were normalized with respect to access resistance they were found to be dependent on the cell size. However, the cell capacitance, which is proportional to the membrane area turned out not to be a precise measure of the cell size. (4) The experimental diffusion rates changed systematically with the aqueous diffusion coefficient and the inverse third root of the molecular weight. Linear interpolation with respect to these quantities provided estimates of diffusion time constants for the diffusion between patch pipettes and the cytoplasm for substances of interest.