On 14 June 1940 state acceptance trials began and it reached at sea level, at , and climbed to in 5.85 minutes. Later, the I-301 reached at with its radiator doors shut making it the fastest M-105P powered aircraft. The I-301 had a fuel capacity of carried in three self-sealing fuel tanks between the wing spars in the wing's center-section giving it a range of . During testing the state specification was increased to a maximum range of and fuel capacity was increased to . Aircraft with this fuel capacity were designated as LaGG-3's while the prototype was re-designated as the LaGG-1. The test flights revealed 114 faults that needed to be fixed but the project was given high priority so most of these faults were to be addressed in production aircraft.