We analyze the atomic–molecular conversion dynamics of a system of condensed lithium isotopes at ultralow temperatures. We have obtained exact analytic solutions that show the existence of aperiodic and periodic regimes of conversion of ⁶Li atoms and ⁶Li⁷Li molecules into the ⁶Li₂ molecule and the ⁷Li atom in the strict resonance conditions. It is shown that the time evolution of atoms and molecules during stimulated conversion is determined by the initial phase difference.