The goal of this study was to determine whether chronic monoxide exposure in the developing heart produces long‐lasting coronary vasculature alterations. One‐day‐old male rat pups were exposed to 500 ppm CO continuously for 30 d, while littermate controls remained in room air (AIR). At 61 and 110 d of age hearts were removed, perfusion fixed, x‐rayed, and processed for analysis of coronary vessel architecture. Body weight (BW) and heart weight (HW) increased with age; the ratio of HW/BW decreased. There were no differences in HW and ventricular dimensions at either age due to prior CO exposure. Morphometric analysis of the fixed hearts from CO‐exposed and AIR rats revealed no significant individual group differences in the number of small (27–114 μm) or larger (>114 μm) vessels in any heart region. The septum (S) in CO rats was an exception: There were more small veins at 67 d of age and more larger veins at 110 d of age. There was a significant increase in the number of small arteries at both ages in the CO rats across all heart regions, and in the smaller veins at 61 d of age. The large vessels in the S at 61 d of age had a significantly greater diameter in CO compared to AIR rats. This was also true for the large arteries in the S and right ventricle (RV) of the 110‐d‐old rats. Taking all heart regions together, the large arteries in CO rats were larger than in AIR rats. Previous CO exposure significantly increased large artery and total cross‐sectional area in the S and RV at 61 d of age, and in RV at 110 d of age. Total cross‐sectional area of veins in the S was also increased. Taking all heart regions together, CO significantly increased small, artery cross‐sectional area at 61 d of age, and small, large, and total artery cross‐sectional area at 110 d of age. With one exception (small veins, 110 d of age), there was no effect of CO on vein cross‐sectional area. These changes resulted in the percentage of total cross‐sectional area contributed by the larger vessels being increased. Pathological examination showed nothing abnormal. The results suggest profound and persistent changes in coronary vessel architecture following chronic neonatal CO exposure.