The expression and distribution of types 1, 2, and 3 inositol 1,4,5-trisphosphate receptor (InsP 3 R) in proliferating, primary cultures of rat aortic smooth muscle were compared to fully developed and differentiated rat aortic smooth muscle. Subtype-specific InsP 3 R antibodies revealed that the expression of type 1 InsP 3 R was similar in cultured aortic cells and aorta homogenate but expression of type 2 and 3 InsP 3 R subtypes was increased 3-fold in cultured aortic cells. The distribution of the type 1 InsP 3 R was located throughout the cytoplasm; type 2 InsP 3 R was found closely associated with the nucleus and at the plasma membrane; type 3 InsP 3 R was distributed predominantly around the nucleus. Alterations in InsP 3 R subtype expression and localization may have important functions in regulating intracellular calcium release around the nucleus when vascular smooth muscle cells switch to a more proliferating phenotype.