Figure 3. Isobolographic representation of the interaction of morphine and clonidine at 60% and 80% levels of response. In both graphs, the axis indicates the mean doses (+/- SEM) of morphine (abscissa) and clonidine (ordinates) that individually produced 60% or 80% of maximal response. Diagonal lines connect equieffective doses of each drug alone and designate additivity. All points were obtained by plotting the values +/- SEM from Figure 1(ED60) and Table 2(ED80). At both levels of effect, morphine and clonidine combined in 1:1 and 1:3 ratios showed antagonism.