On the surface, nothing has changed between Karenin and his wife. Having spoken once of his jealousy and suspicion, however, Karenin can't let it go. He is cold and sarcastic and passive aggressive with her. He thinks that if she wants to mess things up, then it serves her right.
It's surprising that such an important, intelligent man is impotent and deliberately blind at home. He has resolved not to think about his wife's feelings, and in fact does manage not to think about them.
Countess Lydia usually spends time at her country house near Anna's place. (Peterhof is, by the way, the name of the Karenin summer estate.) Countess Lydia has stayed at home and hinted to Karenin about the inappropriateness of Anna's friendship with Betsy Tverskoy and Vronsky.
Karenin defends Anna and turns a blind eye to the fact that other people in society are thinking the same thing that Countess Lydia is thinking.
On the day of the fateful race, Karenin goes out to the country house to see Anna—which he makes a point of doing once a week—and to see the races.
He goes about his business, and then at the end of the day invites his secretary out to the country house with him. Without acknowledging it, Karenin has been trying to have someone else present for meals with his wife to create a buffer between the two.