This thesis describes an investigation of the pattern of cellular activity displayed within the medial prefrontal cortex (mPFC), amygdala and the nucleus accumbens (Acb) during the acquisition of heroin (0.3 mg/kg SC) conditioned place preference. We measured Fos immunoreactivity (IR) in the target regions after the initial and final day of conditioning in four groups that received exposure to different aspects of condition; CS-US, CS only, US only, or noCS-noUS. After a single day of conditioning, neural activity was enhanced in the amygdala and mPFC by the administration of the drug. However, the response to heroin in the mPFC was enhanced by simultaneous exposure to the conditioning environment. In the Acb, there were equivalent increases in activity in all four groups. Importantly, group differences were only found after one day of conditioning, a conditioning period that a subsequent experiment demonstrated not to be sufficient to induce a significant place preference.