Abstract
Rats mated post partum were permitted to nurse 2 pups until day 9, at which time laparotomy was performed and the number of implantation sites counted. Control animals were left to nurse 2 pups or none, while lactation was then accelerated in 4 other groups by the addition of more pups to totals of 3, 5, 7 or 9. On day 19 the mothers were killed and the fetuses counted, weighed and checked for normality. With increased lactational demand the proportion of fetuses maintained decreased. Total fetal weight was decreased significantly only when 7 or more pups were being nursed. Individual fetal weights and normalcy were not affected by accelerated lactation.