Expression of a single copy of the rat obesity fatty (fa) gene may affect energy balance. To test this hypothesis, the effects of zero, one, and two copies of fa on early growth were evaluated, using a molecular genetic method for counting fa alleles inherited by 7- and 14-day-old F2 offspring of a BN/Crl x Crl:ZUC-fa F1 intercross. Litter and sex effects were controlled by multiple-regression analysis, allowing genotype effects on the weights of body, inguinal adipose pads, interscapular brown adipose tissue, and liver to be isolated. At 7 days of age, the fa copy number had linear effects on body and inguinal adipose pads weight. At 14 days of age, the fa copy number had linear effects on body, inguinal adipose pads, and interscapular brown adipose tissue weights and an additional quadratic effect on inguinal adipose pads weight. Thus fa has codominant effects on growth during the first week of life. The recessive effects of fa on growth appear during the second week of life.