Previous research has suggested that horse breeding, with the army as the intended buyer, was an important part of the local agrarian economy in the Roman Dutch eastern river area. Since it is very difficult to trace the origins of horses by traditional archaeozoological methods, strontium isotope analysis was used to investigate the origins of horses in both military and rural sites. These new data are integrated with data on horse frequencies and size to assess the economic importance of horses in rural communities in the eastern river area and further investigate possible supply networks. Both horse frequencies and horse size increase from the Early Roman period onwards, reflecting the significant economic importance of horses in this region. The laser ablation 87Sr/86Sr ratios show evidence for mobility in military horses but not in rural horses.