Both conditioned responses (CRs) and sensitized behaviors induced by addictive drugs are considered to reflect drug-seeking motivation. Based on an excitatory conditioning model of behavioral sensitization, this work hypothesizes that conditioned locomotor activity and locomotor sensitization concomitantly occur using different drug treatment regimens. In the present study, conditioned locomotor activity and sensitized locomotion and stereotypy are assessed with pretreatment of two doses of morphine in a familiar or novel environment. When rats are trained with morphine (3 or 5 mg/kg) in an environment to which the animals are habituated, a CR but not contextual sensitization is induced when tested after 1 week of abstinence. When rats receive the 5 mg/kg dose of morphine immediately after placement into a novel environment, the same results are obtained, but when the drug dose is decreased to 3 mg/kg, both the CR and contextual sensitization are observed. Therefore, the sensitized behaviors, rather than the CR produced by morphine pretreatment, appear to be dependent on the drug treatment regimen and environmental novelty, suggesting that different mechanisms may be involved in the expression of the CR and contextual sensitization. morphine, conditioning, sensitization, locomotion, stereotypy Citation: Liang J, Peng Y H, Ge X X, et al. The expression of morphine-induced behavioral sensitization de-pends more on treatment regimen and environmental novelty than on conditioned drug effects.Repeated intermittent exposure to potentially addictive drugs leads to pathological levels of incentive salience attributed to drugs and drug-associated cues [1,2]. According to the excitatory conditioning model, sensitization is context-specific because it occurs in response to Pavlovian conditioning [3,4]. However, dissociation between the conditioned response (CR) and behavioral sensitization has been found in many studies, e.g. amphetamine and methylenedioxymethamphetamine (MDMA) induced conditioned hyperlocomotion in the absence of prior sensitization [5,6]. By contrast, extinction of the post-sensitization CR did not impede the subsequent expression of contextual sensitization [7,8]. The behavioral effects induced by frequently *Corresponding author (email: zhengxg@psych.ac.cn) abused drugs are significantly impacted by the drug dose and the experimental drug administration design [9−12]. The present study sought to verify the relationship between CR and drug-induced sensitized behaviors using different morphine-treatment regimens by altering the drug doses and the environment novelty.In the present study, when rats were trained with morphine (5 mg/kg) in an environment to which the animals were habituated, a CR occurred, but not contextually sensitized behaviors, when tested after a 1-week abstinence period. The results were similar when the dose of morphine was reduced to a 3 mg/kg pretreatment. This diminished sensitization of locomotor activity did not derive from any "masking effect" of the emergence of stereotyped beh...