The development of tolerance to morphine in the rat

Tolerance to various effects of morphine in the rat can be quantified by means of a shift of semilogarithmic dose-response curves. Tolerance to analgesia (hot plate, acetic acid writhing), catalepsy, and the tilted plane develops in a closely similar manner. Also, the stimulating effects of about 1 mg/kg morphine-HCl tested in an open-field procedure are somewhat less pronounced in chronically treated rats than in naive ones. There is no correlation between tolerance development and the acute ED₅₀ of different tests.