At the behest of his former teacher, Meltzer, Feinstein traveled to Brisk to study under the illustrious Yitzchok Zev Soloveitchik. It did not take long for Feinstein to earn his reputation as the genius in the Brisk yeshiva as well. However, during this time, he became eligible for conscription into the army, so he traveled to Grodno to obtain fake medical forms from a doctor and en route, consulted with Yisrael Meir Kagan (the Chofetz Chaim) in Radin about evading the army. Upon their meeting, Kagan was impressed by Feinstein's broad knowledge of Nezikin, Nashim, and Kodoshim. Feinstein stayed in Grodno for half a year, where he learned from Shimon Shkop. He afterwards returned to Brisk to continue studying under Soloveitchik, spending the summer months and the month of Elul at the Mir yeshivas. When World War II broke out, he traveled to Vilna with other students from the Mir to hear lectures from the Soloveitchik. Whilst in Vilna, Feinstein was lavishly praised by the leading "posek" of the generation, Chaim Ozer Grodzensky.