Figure 1. Isobolographic representation of the interaction of morphine and clonidine combined in a 1:1 proportion, at 60% level of response. The axes indicate the mean doses (+/- SEM) of morphine (abscissae) and clonidine (ordinates) that individually produced 60% of maximal response. The diagonal line connects equi-effective doses of each drug alone, and the point to the right shows the doses of the combination (+/- SEM) that produced the same level of effect. The results demonstrate that the combination is antagonistic.