Pah^enu2 mice, created by chemically induced genetic mutation, are characterized by biochemical phenotypes closely resembling untreated human phenylketonuria (PKU). However, studies conducted in adult Pah^enu2 mice have shown no indices of the severe mental retardation that characterizes untreated PKU. The present experiments explored recognition of novel spatial and nonspatial information in Pah^enu2 mice by two nonassociative tests that do not use explicit reinforcement and avoid lengthy training. Moreover, we evaluated emotional reactivity by the Elevated Plus Maze. Finally, the performance of affected mutants was compared with that of their unaffected and heterozygous littermates and also with that of mice of the C57BL/6 (C57) inbred strain, an increasingly used background for genetic targeted organisms, and with DBA/2 (DBA) mice, known for their nonpathological deficits in spatial learning. The results demonstrated that mutant Pah^enu2 mice are characterized by deficits involving both spatial and nonspatial recognition, that are not related to motor impairment or to high emotional reactivity to novelty. These results indicate that Pah^enu2 mice show pathological cognitive deficits and support their use to test hypotheses about neurodevelopmental disturbances involved in mental retardation.