Functional pain disorders are highly prevalent, but difficult to diagnose and treat. There is strong evidence that dysfunction in central pain modulation contributes to pain amplification and hypersensitivity. An important system in normal and pathological pain is a brainstem pain-modulation circuit that regulates nociceptive transmission through direct projections to the dorsal horn and trigeminal nucleus. The rostral ventromedial medulla (RVM) is the output of this system and facilitates or inhibits pain through two classes of cells termed “ON-cells” and “OFF-cells”, respectively. These cells are defined physiologically based on changes in firing during response to noxious stimulation; ON-cells are activated and have a “burst” in activity, while OFF-cells are inhibited and have a “pause” in ongoing activity. While the role of RVM in nociception has been well-characterized in male animals, very little is known about RVM activity in females. However, chronic pain disorders are more prevalent in females, and animal research has revealed sexual dimorphisms in other parts of this pain-modulation system. Therefore, it is important to characterize RVM activity in females, as sex differences could have clinically relevant implications. Thus, the first aim of this thesis was to compare the basic firing properties and light-evoked stimulus-response curves of RVM cells in female versus male rats.