text: Previous studies have provided very useful measures of relative intrinsic activity for MOPr agonists (Traynor and Nahorski, 1995; Yu et al., 1997; Kovoor et al., 1998; Zaki et al., 2000; Clark et al., 2006; Saidak et al., 2006), however this parameter is not able to differentiate full agonists in terms of efficacy and can be highly dependent upon receptor expression levels, so for these reasons, operational efficacy is a more useful measure.