and single-photon emission computed tomography (SPECT). Functional magnetic resonance imaging (fMRI) is widely used in human subjects because it has much higher resolution and eliminates exposure to radiation. Limitations Although the reinstatement protocols are used frequently in laboratory settings there are some limitations to the validity of the procedures as a model of craving and relapse in humans. The primary limiting factor is that in humans, relapse rarely follows the strict extinction of drug-seeking behavior. Additionally, human self-reports show that drug-associated stimuli play a lesser role in craving in humans than in the laboratory models. The validity of the model can be examined in three ways: formal equivalence, correlational models, and functional equivalence. There is moderate formal equivalence, or face validity, meaning that the model somewhat resembles relapse as it occurs outside of the laboratory setting; however, there is little face validity for the procedures as a model of craving. The predictive validity, which is assessed by correlational models, has yet to be determined for the procedures. There is sound functional equivalence for the model, which suggests that relapse in the laboratory is reasonably similar to that in nature. Further research into other manipulations or reinforcements that could limit drug-taking in non-human primates would be extremely beneficial to the field. Differences between sexes There exists a higher rate of relapse, shorter periods of abstinence, and higher responsiveness to drug-related cues in women as compared to men. One study suggests that the ovarian hormones, estradiol and progesterone, that exist in females at fluctuating levels throughout the menstrual cycle (or estrous cycle in rodents), play a significant role in drug-primed relapse. There is a marked increase in progesterone levels and a decrease in estradiol levels during the luteal phase. Anxiety, irritability, and depression, three symptoms of both withdrawal and the human menstrual cycle, are most severe in the luteal phase. Symptoms of withdrawal not associated with the cycle, such as hunger, are also enhanced during the luteal phase, which suggests the role of estradiol and progesterone in enhancing symptoms above the naturally occurring level of the menstrual cycle. The symptoms of craving also increase during the luteal phase in humans (it is important to note that the opposite result occurs in female subjects with cocaine addiction suggesting that cyclic changes may be specific for different addictive substances). Further, the drug-primed response is decreased during the luteal phase suggesting a time in the