With DNA as a measure of mammary gland involution a 42% decrease in DNA between day 1 of lactation and day 5 of involution was noted. By day 10, DNA decreased 85% and remained essentially constant throughout the the remainder of the period. E (estradiol benzoate) alone appeared to augment retrogression of DNA during initial days of involution, but with E + P (progesterone) this effect was not apparent. With both E and E + P, mammary gland DNA was increased during the later days of experimental period.