THC served as a discriminative stimulus in both rats and mice, as has been shown previously (Järbe and McMillan, 1979; McMahon et al., 2008; Vann et al., 2009; Wiley et al., 1993). In the three groups of THC-trained mice, initial dose-effect curves for THC-associated responding showed considerable overlap across the entire dose range tested. This remarkable similarity occurred despite substantial diversity in several procedural variables between Experiments 1 and 2. While the overall shape of the dose-effect curves for THC-associated responding was also similar across species, two major differences were apparent. The first was the higher training doses used in mice than in rats, with 5.6 to 30 mg/kg used for mice (McMahon et al., 2008; Vann et al., 2009; Wiley et al., 2011) and 1.8 to 5.6 mg/kg used for rats (Järbe et al., 2001; Järbe et al., 2006; Wiley et al., 2004). A second major species difference was the lower baseline response rate in mice than in rats, which occurred regardless of whether the response for mice was a lever press (Vann et al., 2009; Wiley et al.,
