AbstractThe ultrastructure of the mouse adrenal cortex was compared after fixation either by immersion in or by perfusion with buffered glutaraldehyde. The most striking feature of the perfusion fixed cortex was the patency of the capillaries that, by comparison, were collapsed in immersion fixed cortex. In addition, the osmotically sensitive mitochondria and endoplasmic reticulum were swollen and damaged only in immersion fixed tissue. In perfusion fixed adrenal cortices, the profiles of the mitrochondria in all three major cortical zones ranged from ovoid to elongated; their cristae were tubular or tubulosaccular (not lamelliform) and arranged in a parallel fashion. Zonal variations in mitochondria1 ultrastructure in immersion fixed cortex may reflect fixation artifacts and are not suitable criteria for distinguishing the cortical zones. In perfusion fixed adrenal cortex, the intensity of the osmium staining of lipid-filled cytoplasmic vesicles was markedly reduced, particularlyin the iona fasciculata, which by comparison was heavily stained by osmium in the immersion fixed cortex, indicating retention of osmiophilic lipids. This perfusion protocol was developed for use in conjunction with electron microscopic autoradiography for mapping radiolabeled lipoprotein uptake in mouse adrenal glands. (J Histotechnol12:195, 1989.)