['To evaluate the influence of an initial heroin experience under a modified two-chained training schedule on drug-seeking behavior after a long abstinence period.', 'Rats were trained to respond for intravenous heroin (120 microg/kg) under a heterogeneous chained schedule of reinforcement using different responses in the first and second links of the chain. Animals received low-frequency drug administration training for four days and were then subjected to one month of abstinence in their home cages. Heroin-seeking behavior induced by re-exposure to the first chain associated context or discriminative stimuli was assessed after abstinence.', 'Almost all animals could acquire operant skills quickly under the two-chained schedule training for four days, as measured in first active response latency, travel speed and goal-box enter latency. Both first chain associated context and discriminative stimulus could reinstate heroin-seeking behavior after one month abstinence.'] Does initial experience of heroin use under a two-chained operant schedule influence drug-seeking behavior after one month of abstinence?