The effects of chronic treatment of young rats (initially 8 weeks old) with the collagen cross-linking inhibitor, beta-aminoproprionitrile (BAPN), on arterial wall properties were studied. BAPN was added to the drinking water for 8 weeks to produce intakes of 0, 100, 200 and 400 mg/kg/day. Systolic pressure of treated animals did not increase with age as rapidly as that of untreated controls. Weight gain of treated animals was inhibited at the highest (BAPN) treatment level. Passive stiffness of isolated, cylindrical segments of carotid arteries was decreased in BAPN-treated animals in a dose-dependent manner. BAPN treatment had no significant effect on the total collagen or elastin content (hydroxyproline) of carotid arteries. Values of maximum active force development to 10 µM norepinephrine plus 75 mM K<sup>+</sup> were decreased by BAPN treatment in a dose-dependent manner. There was no significant effect of BAPN treatment on total water content or its cellular and extracellular components in the carotid artery. The relative cell volume of carotid arteries was not altered by BAPN treatment, suggesting that the decreased force development was a characteristic of individual cells. These results suggest that BAPN treatment may decrease stiffness by altering secondary characteristics of the connective tissue matrix without affecting connective tissue content. The decreased maximum smooth muscle force development may be responsible for the blood pressure-lowering effects of BAPN.