This study was aimed on development of cross-sensitization with morphine (MO) to methamphetamine (MET). Behavioural responses were measured in the "open field" test. Mice in two groups were pretreated for seven days either with MO at the dose 10 mg/kg/day or with MET at the dose of 2,5 mg/kg/day. On the day 8 all mice were given the "challenge dose" of MET 2.5 mg/kg to evaluate if a cross-sensitization developed. It has been shown that behavioural sensitization was present in both the group pretreated with MET and also in the group pretreated with MO.