Ultrastructural stereological techniques were used to quantitate the phenotype of vascular smooth muscle cells grown in primary culture on a plastic substrate (control), on and within matrices of collagen type I, and on collagen type III, collagen type IV, or basement membrane Matrigel. The volume fraction of myofilaments (Vv myo) of freshly isolated cells at day 0 was 54.2 +/- 2.0%. By day 5 in culture, the Vv myo of cells grown on plastic, on collagen type I, and collagen type III had decreased significantly to 14.2 +/- 2.1%, 16.5 +/- 2.4%, and 16.3 +/- 2.9%, respectively. In contrast, smooth muscle cells grown within matrices of collagen type I, on collagen type IV, or on basement membrane Matrigel had a Vv myo of 30.7 +/- 1.5%, 30.8 +/- 4.2%, and 32.1 +/- 3.1%, respectively. These findings demonstrate that the presence of extracellular matrix components prevents modulation of smooth muscle phenotype early in culture and suggests that the extracellular matrix synthesized and secreted by smooth muscle in the normal vessel wall contributes to the maintenance of their contractile functional state.