The surface-specific dissolution rates ( G-values) of two size fractions of felodipine, a sparingly soluble drug, at different concentrations of polysorbate 80 were compared, using a Coulter Counter Model TAII. The solubilities of the drug at these surfactant concentrations were determined photometrically. The dependence of particle-size on dissolution rate was also evaluated for a number of size fractions of felodipine at a low concentration of polysorbate 80, using the Coulter method. It was shown that the effect of particle fineness was stronger than could be predicted from the differences in initial surface area. The G-values for a number of felodipine qualities were calculated from dissolution data obtained by the USP XXI rotating paddle method at both low and high concentrations of the micellar solutions used. The data delivered by the two dissolution methods were in fairly good agreement. The higher polysorbate concentration gave the same discrimination of the size fractions tested, although higher G-values were obtained.