Recently a number of factors governing the rate of release of drug from plastic matrices were investigated. This study showed that while the experimental results were. Generally in agreement with Higuchi's relationship, they were not always quantitative. The present paper describes a refined experimental procedure for quantitatively studying the various factors. Matrix porosities are determined in two ways so that available and inaccessible pores can be differentiated. Difision coefficients are independently determined. The matrix tortuosity can now be quantitatively determined independently of the solid drug release rate data. In addition to these experimental refinements, the limitations of the theory are reviewed and some useful modifications proposed.