Fig. 3. Isobolographic representation of the interaction of morphine (MS) and clonidine (CL) (1:1 ratio) at 60% level of response (ED60 or plateau). The straight line indicates the zero interaction isobole at the ED60value, and the points drawn on the left and right of the additive line show synergy and antagonism, respectively. Points on the axes represent isoeffective doses of morphine and clonidine individually, whereas points on the left and right of the isobole represent isoeffective combinations derived form the dose–response curves (DR) or the plateau (PT). (Top ) Naive animals; (bottom ) morphine-tolerant mice.