During pregnancy, the ovine uterine artery changes from a low- to a high-stress artery. We investigated the hypotheses that the increased stress reflects alterations in vessel wall cellularity, smooth muscle cell contractile protein contents, or activation properties. Uterine artery diameter increased during pregnancy, whereas the fractional cellular composition and thickness of the muscularis were unchanged. Results of morphometry suggest that vessel growth is associated with cell elongation. Uterine arteries from pregnant ewes had greater protein contents than those from nonpregnant ewes (104 vs. 69 mg/g, respectively); there were corresponding increases in the absolute cellular contents of actin and myosin. While the fraction of light chain phosphorylated in response to phenylephrine was unaltered, the total amount of myosin light chain phosphorylated per gram wet weight increased significantly during pregnancy. In addition, the distribution of myosin heavy chain isoforms was also altered during pregnancy. The increased stress observed in the uterine artery during ovine pregnancy reflects, in part, increases in cellular contractile protein concentrations associated with hypertrophy.