Figure 2. Isobolographic representation of the interaction of morphine and clonidine at 20% and 50% levels of response. In both graphs, the axis indicate the mean doses (+/- SEM) of morphine (abscissa) and clonidine (ordinates) that produced 20% or 50% of maximal response when administered separately. Bold diagonal lines connect equieffective doses of each drug alone and designate additivity (no interaction). All points in the graphs were obtained by plotting each of the paired values +/- SEM found in Table 2(morphine:clonidine in different proportions), and concave-up lines are the isoboles of morphine and clonidine at the indicated levels of response.