<P> In the second season, Leonard began a stable relationship with a physician named Stephanie Barnett . Although the relationship went well, even earning Sheldon's approval, Leonard felt uncomfortable with Stephanie moving in with him, and the relationship ended . </P> <P> On another occasion, when Leonard's mother was visiting, he and Penny came very close to having sex, but Leonard ruined the moment by suggesting that they were burying their lifelong issues with their parents by doing so, which offended Penny . In the second - season finale, when Leonard, Sheldon, Howard, and Raj were due to leave for a three - month scientific expedition to the North Pole, Penny privately expresses sadness, and wishes that he would not leave . </P> <P> In the third season premiere, Leonard and Penny finally started a romantic relationship and had intercourse for the first time . However, the relationship ended after eight months, after Leonard told Penny he loved her and took umbrage at her inability to reciprocate, which led to friction that was exploited by guest star Wil Wheaton who appeared as a fictionalized version of himself, who observed and exacerbated it in order to break them up in the middle of a bowling match Wheaton's team was having against Leonard, Penny, Sheldon, Howard and Raj . </P> <P> Shortly after his breakup with Penny, in "The Plimpton Stimulation", Leonard has a one - night stand with Dr. Elizabeth Plimpton (Judy Greer), a physicist he admires, when Sheldon invited her to stay in their apartment for a brief visit . This encounter causes some tension between Leonard and Penny, who is somewhat judgmental of Leonard's doing so . However, the two later resolve their feelings and their friendship continues . </P>

Why did penny and leonard break up in season 3