Three stables between Daventry and Long Buckby were destroyed in a fire.
A small outbuilding was also gutted in the fire which happened on the B4036 between Daventry and Long Buckby at about 1am on Friday, January 8.
Officers investigating the fire would like to speak to anyone who saw the blaze or who knows anything about how it started.
Witnesses, or anyone who knows anything about this incident can call Northamptonshire Police on 101.
Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111 or use the anonymous online form at www.crimestoppers-uk.org.