The in situ expression of cytochrome P450 3A- (CYP3A) like proteins in hepatic and extrahepatic tissues from a marine mammal, pilot whale (Globicephala melas), was investigated. Polyclonal antibodies (PAb) raised against either rat CYP3A1 or trout CYP3A27 both recognized a microsomal protein band in liver, lung, kidney and heart. The protein band observed in liver and lung had slightly lower molecular weight than that observed in kidney and heart, suggesting the existence of two CYP3A forms in pilot whale. Immunohistochemical analyses showed strong CYP3A-staining in hepatocytes, bile duct epithelial cells, bronchial epithelial cells, in primordial- and primary follicles and their surrounding zona glomerulosa. Moderate to strong CYP3A staining was seen in smooth muscle-like cells of large arteries and arterioles in all organs examined. Mild to moderate staining was evident in alveolar epithelial cells and in kidney tubular epithelial cells. Weak staining was seen in glomerular epithelial cells and in seminiferous tubular epithelial cells.