The same administration schedule of morphine leads to different degrees of tolerance in different test situations. The most intense one was found to depressing effects. Slight tolerance to stimulating behaviour, observed in an open field stiuation at low doses, develops, too. There is a more marked tolerance to the depression of stimulating effects, thus the bell shaped dose-response relationship is broadened and shifted somewhat towards higher doses. All effects observed in dependent animals were also seen after acute morphine administration. Whereas tolerance in all tests was dependent on dose and administration time this was not the case for dependence determined by naloxone challenge on days 10 and 20 of administration. Thus, in the rat no correlation exists between naloxone provoked withdrawal and tolerance.