Glycogen phosphorylase (EC 2.4.1.1) may be assayed in the glycogen degradation direction by a continuous spectrophotometric method. The formation of glucose 1-phosphate from glycogen and phosphate produces a controlled change of pH which can be measured by the changes in absorbance of phenol red added to the system. The procedure may be conveniently applied to a stopped-flow spectrophotometer to measure the rate of the reaction. Therefore the activity of the enzyme may be determined at low conventional concentrations and, by the same technique, at high enzyme concentrations approaching those supposed to exist in vivo.