In light of previous research showing that different types of relationships affect levels of testosterone in men, this study examined whether categorizing relationship types according to relationship length can shed further light on variations in levels of testosterone. Salivary testosterone samples were obtained from a sample of men and details about their relationship status, sociosexual orientation, extra-pair sexual interest, and their perceptions of their relationships were recorded. Using a median split analysis, participants who indicated that they had been in their relationship for less than 12 months were categorized as being in “new relationships” and those in longer relationships being categorized as in long-term relationships. Results showed that levels of testosterone of single men and men in new relationships did not differ, but both had significantly greater levels of testosterone than men in long-term relationships. Differences in levels of testosterone were unrelated to sociosexual orientation and extra-pair sexual interest. These findings support the evolutionary explanation of levels of testosterone in men varying in accordance with their internal motivation to seek new potential mates.