Si{nchez,G. F.,F.,Montaldo,V.,H.andhdrez,L.,A. lgg4.Environmentalandgeneticeffectsonbirthweightingradedup goat kids. Can. J. Anim. A linear model analysis was carried out to study the effect of sex, litter size, age of dam, year, birth period and genetic group on birth weight in goats. Genetic groups consisted of graded-up goats from backcrosses of local goats with Alpine, Granadina, Nubian, Saanen and Toggenburg sires, classihed as low grade (from 1/2 to 3/4 ofexotic genes) and high grade (over 3/4 ofexotic genes). Heritabilities were also obtained for low-and high-grade kids.The data concern 4623 records from a farm under a stall feeding system in the north of Mexico. All of the main fixed effects but only the genetic group x birth period interaction were significant (P < 0.05). Males were heavier than females and singles were heavier than double and multiple-born kids. Birth weight means increased as dam age increased. Birth weight means for high and low grade kids were, respectively, 3.3 and 3.2 kg in Alpine kids, 3.1 and 3.2 in Nubian,2.7 and 2.9 in Granadina, 3.3 and 3.3