Comparative investigation of analgesic (writhing test) and reinforcing (conditioned place preference) effects of 11 opioids over a wide range of doses was performed in mice. On the basis of the dose-response curves analysis, parameters of potency and efficacy of the reinforcing effect were determined. In one group of analgesics (etorphine, fentanyl, buprenorphine, morphine, promedol) ED50 in both tests were approximately equal. The other compounds (pentazocine, nalbuphine, nalorphine, butorphanol, U50,488H) demonstrated dissociation of two effects. The new indices (addictive index and safety index) for the prediction of analgesics' as well as other psychotropic drugs' abuse potential at the preclinical stage of their study have been suggested.