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Given the evolutionary importance of social ties for survival, humans are thought to have evolved psychobiological mechanisms to monitor and safeguard the status of their social bonds. At the psychological level, self-esteem is proposed to function as a gauge—‘sociometer’—reflecting one’s social belongingness status. At the biological level, endogenous opioids appear to be an important substrate for the hedonic signalling needed to regulate social behaviour. We investigated whether endogenous opioids may serve as the biological correlate of the sociometer. We administered 50 mg naltrexone (an opioid receptor antagonist) and placebo in a counterbalanced order to 26 male and female participants on two occasions ∼1 week apart. Participants reported lower levels of self-esteem—particularly self-liking—on the naltrexone (vs placebo) day. We also explored a potential behavioural consequence of naltrexone administration: attentional bias to accepting (smiling) faces—an early-stage perceptual process thought to maximize opportunities to restore social connection. Participants exhibited heightened attentional bias towards accepting faces on the naltrexone (vs placebo) day, which we interpret as an indicator of heightened social need under opioid receptor blockade. We discuss implications of these findings for understanding the neurobiological underpinnings of sociality as well as the relationship between adverse social conditions, low self-esteem and psychopathology.