The defensive glands of tenebrionid beetles produce a mixture of benzoquinones and hydrocarbons. These reactive toxicants are manufactured within a complex morphological system, consisting of secretory cells, secretory vesicles within these cells, and cuticular organelles and tubules which carry the products to the gland reservoir. Histochemical and thin-layer chromatographic techniques were employed to follow the process of toxicant production within these cells. Chromatographic evidence indicated the presence of both p-diphenols and their glucosides in extracts of the secretory cells, which suggested that quinones are generated from phenol glucosides by hydrolysis and subsequent oxidation. Histochemical tests demonstrated concentrations of phenolic materials in the cytoplasm of one cell, type 2a, and on the basis of their sensitivity to emulsin, it was concluded that these are phenol glucosides. Glucosidase activity, as shown by a modified bromonaphthol-glucoside technique, was associated with the margins of the vesicle in the same cell type. The cuticular organelle, within this vesicle, yielded positive reactions for a phenolase both with dopa and with N-phenyl- p-phenylene diamine as substrates. Along the efferent tubule which runs from this organelle to the gland reservoir, a haemoprotein peroxidase was detected by the benzidine peroxidase test. Each of these steps in quinone production was associated with a particular morphological compartment, and it is apparent that the cytoplasm of the secretory cell is never exposed to the toxic product of the secretory unit. Lipid reserves, which are possible precursors of the hydrocarbon components in the secretory end-products, were found in all secretory cells. However, the densest deposits were concentrated in the type 1 units, and in these units, a paraoxon-resistant carboxylic esterase was demonstrated with indoxyl substrate, along the margins of and within the secretory vesicle. Deposits of unsaturated lipids were detected in the cuticular organelle of this vesicle.