Dragon Ball Z - episode 32 - 16 Jan, 1990

By creating an artificial moon with a specialized ball of energy completed with Blutz Waves, Vegeta transforms into a Great Ape. But unlike most other Saiyans, he has complete control of himself while in this transformed state. Upon seeing Vegeta's ape form, Goku remembers transforming into a Ape as a child, realizing he unintenionally killed his Grandpa Gohan. Luckily, Goku has one last attack that could finish Vegeta: the Spirit Bomb. As Gohan and Krillin return to the battlefield, Goku is able to create the bomb. Vegeta, however, is able to prevent Goku from using it. With no energy left, Goku is now at the mercy of Vegeta. (Source: Wikipedia)