The enantiomers of eseroline bind to opiate receptors of rat brain membranes with equal affinities and show opiate agonist properties as inhibitors of adenylate cyclase in vitro. However, only (-)-eseroline shows potent narcotic agonist activity in vivo, similar to that of morphine. Neither (-)-noreseroline, (+)-eseroline, nor the open dihydroseco analogue (-)-8 shows analgetic effects in vivo. The structure of rubreserine being a resonance hybrid of an o-quinone with its zwitterionic mesomer is confirmed by solid-state X-ray diffraction analysis.