Animals have the fascinating ability to play a key role in their own physiological responses, whether consciously or subconsciously. The body's response to and production of opiates are of particular interest, as a more nuanced understanding of opiates' role in the maintenance of emotional stability, well-being, and behavior has emerged. Individual differences in how exercise alters endorphin (endogenous opiate) levels were investigated by studying opiate activity induced by wheel running in selectively bred rats. Rats from the Occidental High-Saccharin (HiS) and Low-Saccharin (LoS) lines were used. Females were used because females, particularly LoS females run far more than their male counterparts. The relationship between increased exercise and endogenous opiate levels was explored through use of the pawlick latency test. The pawlick latency test is an empirical method of measuring pain from which functional opiate levels can be tentatively inferred: Longer latencies to lick a paw on a hotplate indicate reduced pain sensitivity, commonly (though not uniquely) mediated by greater endogenous opiate activity. If running increases endogenous opiate activity, the more hyperactive rats - presumably LoS rats - should be less sensitive to pain, and thus have longer pawlick latencies. In further investigation, I will employ the opioid antagonist naloxone as a tool in obtaining pharmacological evidence that opioids mediate any behavioral effects.