Fig. 4. Log dose–response curves to morphine alone (circles) and in the presence of a fixed dose (0.02 mg/kg) of clonidine (triangles) in animals tolerant to morphine. In the ordinates we have represented percent inhibition of gastrointestinal transit, and in the abscissa, doses in milligrams per kilogram. Each point is the mean ± SEM of six to eight mice.