The purpose of this study was to compare the pharmacological characteristics and actomyosin content of arterial and venous tissue at different times during development. Rings of arteries (femoral, renal, carotid, pulmonary) and veins (saphenous, pulmonary, jugular) were obtained from 1 wk, 1 month, and adult dogs, mounted at their optimal length for force development and the contractile response to potassium chloride and phenylephrine determined. The strain at optimal length was less at all ages in pulmonary artery and pulmonary and jugular veins than in other vessels. All vessels exhibited an increase in maximum contractile response with development but the increase was greater for phenylephrine. In general, the magnitude of the maximum response of the jugular and pulmonary veins and pulmonary artery was less than other vessels at all ages. The sensitivity (half maximum response) either increased or was unchanged in arteries with development, while in the veins it either decreased or was unchanged. The relaxant effects of verapamil and isoproterenol were determined on potassium chloride contracted vessels. Arterial tissue was minimally responsive to isoproterenol at all ages while venous tissue either increased its responsiveness (saphenous, pulmonary) with development or remained highly responsive (jugular). Verapamil, unlike isoproterenol, was an effective relaxant of all vessels. The actomyosin content (mg/mm) of femoral and renal arteries and saphenous and jugular veins increased with development but this increase was accompanied by a parallel increase in total protein so that the ratio (actomyosin/total protein) was unchanged. In jugular veins from adult dogs this ratio was smaller than in arterial tissue. In general, it can be concluded that arterial and venous tissues increase their maximum contractile response during maturation. Because the maximum response to agents with different mechanisms of action (potassium chloride, phenylephrine) increased at different rates, the increase must be due to more than a quantitative increase in contractile material, possibly to differences in the rate of maturation of their respective excitation-contraction coupling processes. However, when maturational changes in other characteristics are compared, differences are observed between arteries and veins as well as between vessels within a given class indicating intervessel heterogeneity in maturation.