Abstract In order to investigate the relationship between the biochemical pathways that characterize contraction and cell growth, we have studied both contraction, mitogenesis and protein synthesis induced by the vasoactive neuropeptides, substance P (SP), calcitonin gene related peptide (CGRP) and vasoactive intestinal polypeptide (VIP) on four different established vascular and non-vascular smooth muscle cell lines. Contraction in vitro was evaluated by light microscopy and recorded photographically. Mitogenesis and protein synthesis were evaluated by [ 3H]-thymidine incorporation into cells and [ 3H]-amino acid incorporation into trichloroacetic acid precipitated materials, respectively. SP stimulated mitogenesis of A7r5 cells (embryonic rat aorta), but failed to induce significant contraction of these cells, whereas, SP induced contraction of cultured adult rat vascular smooth muscle cells (VSMC), but failed to stimulate mitogenesis. CGRP and VIP stimulated mitogenesis and protein synthesis, respectively, of DDT 1MF-2 cells (hamster vas deferens), but neither induced contraction of this cell line. All three neuropeptides showed no effect on BC 3Hl (mouse smooth muscle-like) cells. These results suggest that neuropeptides with vasoactive properties modulate different stages of cellular mitogenic responses which may be regulated by the degree of maturation of smooth muscle cell.