Male and female pups were cross-fostered into litters of 6 (circles) or 12 (triangles). From day 4 to 14, half the pups in each litter received leptin (80 ng/g/d, open symbols or bars), while littermates received an equal volume of normal saline (10 ml/kg/d, solid symbols or bars). While mice fostered in litters of 12 had decreased neonatal weights (F(1,740)=52, *P<0.001), leptin administration did not acutely influence pup weight (F(1,720)=1.4, P=0.24) (A). Plasma leptin levels were influenced by an interaction between litter size and leptin (B; F(1,7)=67, *P<0.001 versus litters of 6 and †P<0.001 versus saline).