Juvenile hormone (JH) is an important insect hormone known to have many effects on development, reproduction, and behavior in both solitary and social insects. This protocol describes how to quantify in vitro biosynthesis rates from excised corpora allata (CA), the glands responsible for JH production. Excised glands are incubated with radiolabeled methionine, resulting in the production of radiolabeled JH III, which can then be quantified. This protocol is most useful when quantification of the activity of the glands, rather than circulating JH levels, is desired. Its difficulties lie primarily in the ability to remove and handle the glands correctly.