Opioids are implicated in social attachments, but their role in avian pair bonds is not well understood. The present study tested the effects of naloxone, an opioid antagonist, on pairing using both a forced-choice and a mixed-sex aviary paradigm. First, three doses of naloxone were systemically administered in males using a repeated measures forced-choice design, partner preference formation was tested on the second day. Males treated with 20 mg/kg sang less undirected song. Males treated with 10 mg/kg of naloxone sang less to the familiar partner than when treated with saline and were less likely to form a partner preference than were other treatments. In females, 10 mg/kg of naloxone in a forced-choice paradigm increased preference for the unfamiliar over the familiar male. Finally, males and females were administered either naloxone (10 mg/kg) or saline in a mixed-sex aviary. In females, naloxone increased pairing behaviours, but had no other effects in either sex. Our findings suggest that the effects of naloxone on pairing-related behaviours are context-dependent; male–male competition may decrease the effects of naloxone on male song and a choice of mates may increase affiliation in females in a semi-naturalistic paradigm, and increase preferences for an unfamiliar partner in a forced-choice paradigm. Our findings highlight the importance of using multiple paradigms to test mechanisms of behaviour. These findings contribute to our understanding of the mechanisms of monogamous relationships and suggest that opioids play a role in male courtship, female affiliation and partner preferences in both sexes of zebra finches, but that context is important.