In isolated cell systems, nitric oxide synthase (NOS) activity is regulated by caveolin (CAV), a resident caveolae coat protein. Because little is known of this interaction in vivo, we tested whether NOS and caveolin are distributed together in the intact organism. Using immunohistochemistry, we investigated the localization of constitutive neuronal (nNOS) and endothelial (eNOS) enzyme isoforms along with caveolin-1 (CAV-1) and caveolin-3 (CAV-3) throughout the systemic vasculature and peripheral tissues of the hamster. The carotid artery, abdominal aorta, vena cava, femoral artery and vein, feed artery and collecting vein of the cheek pouch retractor muscle, capillaries and muscle fibers of retractor and cremaster muscles, and arterioles and venules of the cheek pouch were studied. In endothelial cells, eNOS and CAV-1 were present throughout the vasculature, whereas nNOS and CAV-3 were absent except in capillaries, which reacted for nNOS. In smooth muscle cells, nNOS and CAV-1 were also expressed systemically, whereas eNOS was absent; CAV-3 was present in the arterial but not the venous vasculature. Both nNOS and CAV-3 were located at the sarcolemma of skeletal muscle fibers, which were devoid of eNOS and CAV-1. These immunolabeling patterns suggest functional interactions between eNOS and CAV-1 throughout the endothelium, regional differences in the modulation of nNOS by caveolin isoforms in vascular smooth muscle, and modulation of nNOS by CAV-3 in skeletal muscle.