Summary Total DNA of rat mammary glands per 100 g of body weight increased from 2.77 mg to 7.89 mg during pregnancy, a 184.8% increase, and total DNA increased from 4.76 mg to 17.19 mg, a 261.1% increase. Total RNA increased from 3.06 mg to 15.44 mg during pregnancy and these increases paralleled those of total DNA. RNA/DNA ratio increased from 0.64 to 0.90 as gestation advanced.