The conversion of exogenous 3H-farnesenic acid to 3H-methyl farnesoate and 3H-C16 juvenile hormone (JH) has been followed in the corpus allatum (CA) cells of the desert locust Schistocerca gregaria by means of electron microscopic autoradiography. Aerobic and anaerobic 'chase' incubations have been used to modify the quantities of these three compounds within the CA cells. Under all incubation conditions, radiolabel is found associated almost exclusively with the subcellular membrane systems - smooth endoplasmic reticulum (SER) and Golgi elements - and with the mitochondria. CA cells are probably similar to vertebrate steriod-synthesizing cells in that the secretory product is synthesized in the SER and mitochondria. Radiolabel was found to be present in all cells of the CA suggesting that all cells are capable of at least the final two stages of JH biosynthesis (the esterification and epoxidation of 3H-farnesenic aid). This indicates that JH biosynthesis may be regulated through changes in the biosynthetic capabilities of individual cells rather than through changes in the total number of cells engaged in biosynthesis. Radiolabel was not observed to be associated with any distinctive cellular product, a result which provides additional evidence for the suggestion that the release of JH from the CA is governed by laws of simple physical diffusion.