Changes in arterial blood pressure (ABP) were studied in freely moving rats during the development and performance of operant IV heroin self-administration (SA) behavior (100 micrograms/kg/injection). Heroin SA was accompanied by bidirectional, phasic ABP fluctuations in the absence of significant alterations in long-term, basal ABP levels. In trained rats, ABP gradually increased starting 5 min before each lever-press for the drug, reached a peak at the moment of lever-press and abruptly decreased after heroin infusion. This biphasic pattern corresponded to a preresponding behavioral activation followed by a postheroin sedation. These ABP fluctuations were absent during the initial heroin self-injections in drug-naive rats and during the first self-injections of a session in trained rats. A slight hypo- and hypertension were seen in these cases, respectively. The postdrug ABP decrease became more pronounced after rats received a double dose of heroin. With training, nonreinforced lever-presses and sound stimulation previously associated with heroin self-injections also significantly decreased ABP. Thus, a gradual ABP increase appears to be an essential correlate of drug-seeking and -taking behavior, while a subsequent ABP decrease may be related to the alleviation of these behaviors by heroin and possibly correlate to its rewarding (euphorigenic) action.