A growing body of evidence indicates that G-protein-coupled receptors undergo complex conformational changes upon agonist activation. It is likely that the extracellular region, including the N terminus, undergoes activation-dependent conformational changes. We examined this by generating antibodies to regions within the N terminus of micro-opioid receptors. We find that antibodies to the midportion of the N-terminal tail exhibit enhanced recognition of activated receptors, whereas those to the distal regions do not. The enhanced recognition is abolished upon treatment with agents that block G-protein coupling or deglycosylate the receptor. This suggests that the N-terminal region of mu receptors undergoes conformational changes following receptor activation that can be selectively detected by these region-specific antibodies. We used these antibodies to characterize micro receptor type-specific ligands and find that the antibodies accurately differentiate ligands with varying efficacies. Next, we examined if these antibodies can be used to investigate the extent and duration of activation of endogenous receptors. We find that peripheral morphine administration leads to a time-dependent increase in antibody binding in the striatum and prefrontal cortex with a peak at about 30 min, indicating that these antibodies can be used to probe the spatio-temporal dynamics of native mu receptors. Finally, we show that this strategy of targeting the N-terminal region to generate receptor conformation-specific antisera can be applied to other G(alpha)(i)-coupled (delta-opioid, CB1 cannabinoid, alpha(2A)-adrenergic) as well as G(alpha)(s)-(beta(2)-adrenergic) and G(alpha)(q)-coupled (AT1 angiotensin) receptors. Taken together, these studies describe antisera as tools that allow, for the first time, studies probing differential conformation states of G-protein-coupled receptors, which could be used to identify molecules of therapeutic interest.