Diet and reproduction are two important characteristics in animals that can be related to environmental conditions and time of year. In this study, we investigated these two parameters in a viviparous lizard, Liolaemus crepuscularis, from the arid Prepuna of northwestern Argentina, for two months of the austral spring (October 2009 and November 2010) and one month at the beginning of the fall (March 2010). Formicidae was the main prey item, similar to what was found in other Liolaemus species; but plant material was also very important, especially in the latter month when lizards consumed the greatest amounts of food. In October 2009 and November 2010, testicular volume in males was low whereas females were gravid. In March 2010, males showed a great increase in testicular volume and females showed vitellogenic follicles, indicating reproductive activity. A negative correlation was found in females between amount of food consumed and advancement of pregnancy. Females ate very little while gravid, possibly in part due to the space occupied by embryos and increased difficulty in foraging, as well as predation vulnerability. No association was found in males between food consumption and testicular volume.