Although the mechanisms remain unknown, maternal core temperature (T-c
) decreases near term of pregnancy and is increased throughout lactati
on in rats. The purpose of our present experiments was to determine wh
ether pregnancy and lactation shift the thermoneutral zone of rats and
to investigate whether the changes in maternal T-c during pregnancy a
nd lactation result from ''forced'' or ''regulated'' thermoregulatory
responses. Conscious, chronically instrumented nonpregnant and pregnan
t and lactating rats were studied both in a thermocline (a chamber wit
h a linear temperature gradient from 12 to 36 degrees C) and in a meta
bolic chamber to determine the influence of pregnancy and lactation on
selected ambient temperature as well as the thermoregulatory response
to changes in ambient temperature. We found that selected ambient tem
perature, oxygen consumption, and thermal conductance did not change i
n rats studied in a thermocline as T-c decreased near term of pregnanc
y. There was, however, a downward shift in the thermoneutral zone of r
ats studied in a metabolic chamber near term of pregnancy. During lact
ation, selected ambient temperature decreased in rats studied in a the
rmocline as oxygen consumption and T-c increased. The thermoneutral zo
ne of lactating rats was not different from that of nonpregnant animal
s. Thus our data provide evidence that the decrease in T-c near term o
f pregnancy in rats results from a regulated thermoregulatory response
, whereas the increase in T-c during lactation results from a forced t
hermoregulatory response.