Reproductive and fat body cycles were studied in a population of viviparous lizards (Sceloporus grammicus) from the Teotihuacán area, state of Mexico between October 1984 and September 1985. Males reached sexual maturity at a snout-vent length (SVL) of 44.5 mm and at an age of 6 to 7 mo; females reached sexual maturity at 44.1 mm SVL and at an age of 8 to 9 mo. Testicular mass of adult males began to increase in size in May and peaked during July, August, and September. Maximum testicular growth was associated with precipitation, but not with temperature and photoperiod. Enlarged vitellogenic follicles appeared during October, and females contained embryos in stages 1 through 8 during November. Females had embryos in later embryonic stages 29 through 40 from January to April. Vitellogenesis and embryonic development were associated with increasing photoperiod, temperature, and precipitation. Mean litter size was 5.09 ± 0.24 (2 to 9). Mean RCM was 0.338 ± 0.024 (0.015 to 0.80). Litter size and RCM were positively correlated with female SVL.