Lipase activity in homogenates of guinea-pig adrenals was studied under conditions which exclude the hormone-sensitive lipase/cholesterol ester hydrolase. Antibody inhibition and chromatography on heparin–Sepharose showed that most of the activity was due to lipoprotein lipase (LPL), and that there was only a small amount of hepatic lipase activity. Northern blot analysis of total RNA demonstrated the same three adrenal LPL mRNA species (1·8, 3·1 and 3·5 kb) as were found in adipose tissue and heart. Hence, at least part of the LPL activity in adrenals is due to enzyme synthesized within the tissue. Immunolocalization showed that LPL was associated with the endothelium of blood vessels throughout the gland. In addition, there was cytoplasmic immunoreaction, suggesting that lipase was synthesized in a subpopulation of cells in the transitional zone between the fasciculata and reticularis layer of the cortex, particularly over lipid-filled cells. There was also intense immunofluorescence over scattered cells in the adrenal medulla. Treatment with an ACTH analogue depot (20 IU, i.m.) for 11 days induced a 12-fold increase in serum cortisol and increased adrenal weight 2·2-fold. The treatment induced increases in LPL mRNA (about twofold), LPL activity and in the number of cells in the adrenal cortex which gave an immunoreaction for LPL.