The effect of pregnancy on lactation was studied during the third week of l
actational pregnancy in postpartum pregnant rats with a delay in implantati
on of only 1 day (1d-LP rats). In an experimental design in which the suckl
ing litter was prevented from consuming solid food, lactational performance
was estimated by weighing the ten-pup suckling litters on days 16-21 of la
ctation or by measuring maternal weight loss after a nursing spell on day 2
1. In 1d-LP rats, food consumption as well as lactational performance was l
ower than it was in nonpregnant lactating rats (L rats) and pregnant-lactat
ing rats with a normal long delay of implantation of at least 6 days (LP ra
ts). The time spent by the pups sucking at the nipples was not different am
ong the three groups, but the number of milk ejections was diminished in 1d
-LP dams. Restriction of daily food supply during days 16 to 21 of lactatio
n diminished lactational performance more strongly in 1d-LP rats than it di
d in L rats; 1d-LP rats conserved protein stores and mobilized fewer minera
ls than did L rats. The weight and composition of the litter in utero were
not affected by the food restriction. Ln pregnant-lactating rats (LP and 1d
-LP rats), the number of early resorptions was increased in comparison with
pregnant rats, showing that lactation can affect the earlier stages of pre
gnancy. It was concluded that late pregnancy does not affect nursing behavi
our, but suppresses lactation by restricting maternal food intake and mobil
ization of maternal stores. Measurements in serum indicate a causative role
for oestradiol, but not for leptin.