ABSTRACT: A goal of neurophysiology of the mesolimbic system is to determine the activity patterns within the regions in the prefrontal cortex, ventral neostriatum, and amygdala that regulate behavioral patterns to seek rewards. A new technology has been introduced in which arrays of microwires are implanted in different brain regions while activity patterns of ensembles of neurons are recorded for long periods of time during freely moving behaviors. Multichannel instrumentation and software is used for data acquisition and analysis. An initial hypothesis was that neural signals would be encountered in the nucleus accumbens and associated regions specifically related to reward. However, an initial study of neural activity and behavioral patterns during a simple lever press for intravenous cocaine (1 mg/kg) revealed that phasic excitatory or inhibitory neural activity patterns often appear prior to the reward phase. Individual neurons throughout the mesolimbic system appear to code information specific to sensory and motor events, tones, or lever presses in the chain of tasks leading to all rewards so far studied. Different spatial temporal patterns also appear within the same neural populations, as reward is changed from injected cocaine to heroin, from ingested pure water to ethanol in water or sucrose. Overall, patterns of activity for each neuron are found to shift dynamically during the operant task as changes are made in the target reward. Significant shifts in activity of mesolimbic neurons that are unrelated to specific sensory-motor events also appear during complex sessions, such as during a bout of ethanol consumption to reach satiation or during progressive ratio tasks with increasing difficulty. An emerging hypothesis is that some candidate neural elements in the mesolimbic system code the anticipated reward, whereas others serve internal logic functions of motivation that mediate extinction or resumption of specific goal-directed behaviors.