Petritsky comes in to their shared cottage, and tells Vronsky that their colonel is expecting him.
Vronsky asks why music is playing.
Petritsky tells him that Serpukhovskoy has arrived (that's the childhood friend of his who has just gotten two promotions and a decoration). Having decided to sacrifice his ambition for his love for Anna, Vronsky no longer feels any sort of jealousy. In fact, he's looking forward to catching up with his old friend.
Debauchery ensues at the party for Serpukhovskoy. Particularly notable is a keg of vodka.
Vronsky and Serpukhovskoy have a conversation that turns to ambition versus women. Serpukhovskoy argues that Russia needs men like Vronsky (i.e., men of independent means who are harder to buy off, as opposed to agitators like the communists) and that women are the primary obstacles to men advancing in their career. Marriage, Serpukhovskoy argues, is the only answer.
According to Vronsky, Serpukhovskoy has simply never been in love.
Serpukhovskoy says that's possible, but tells Vronsky to remember that women are more materialistic than men, and that men tend to view love as something vast and abstract.
A footman comes up with Betsy's note. As soon as he reads it, Vronsky promptly says he has a headache and must go home.