In season three, he is revealed to have been dating , one of the medical examiners. His relationship with Peyton has not been easy; his insistence that they keep their relationship under wraps at the lab conflicts with her desire to be less secretive about them. A particularly tense moment occurs during episode 3.11 "Raising Shane" when, during a debate about the state of their relationship, Mac accidentally calls Peyton "Claire", which causes her to walk out on him (somewhat ironic in light of Peyton being played by actress Claire Forlani). The two reconcile with a hug in the lab in the next episode (3.12 "Silent Night"). Later, by episode 3.16 "Heart of Glass", Mac and Peyton have evidently openly acknowledged their relationship, when they show up together at a crime scene on Mac's day off because Peyton is on call as the Medical Examiner. Danny Messer, who has already arrived on scene as the CSI, says he thought Mac had the day off. Mac confirms this, but says that Peyton was on call & he is with her. Danny then asks Mac, "So am I the last one to hear about this?" To which Mac replies, "I guess so". Danny asks if Don Flack knows, and Mac confirms that he does. This seems to indicate that Danny is indeed the last to know about the relationship, and no one at the lab has expressed any reservations about it. In 3.24 "Snow Day", Peyton invited Mac to go with her to England while she's there for a medical examiners conference. She wants him to use ten days of the seven weeks of vacation time he has accumulated. At first Mac is uncertain if he wants to go, but after the events in the episode take place, he informs his teammates that he is going to England with Peyton. However, Mac returns to New York without Peyton at the start of season 4, and at the end of the 4th episode, "Time's Up", he receives a letter from her saying that she has decided to stay in London with her family and that a long-distance relationship would never work, because "however close we may be, there would always be an ocean between us." Mac seeks solace in his bass guitar in a jazz bar.