The fetus' experience of birth derives from a sequence of stimulation provided by the mother's labor contractions, her licking and handling, and the contrasting environmental conditions of the uterus and outside world. In the present investigation, Day 21 fetal rats were externalized from the dam's body; subjects in one uterine horn were compressed by simulated uterine contractions while control subjects in the opposite horn were not compressed. All pups were Cesarean-delivered, stroked, and exposed to a thermal environment simulating either room (21 degrees C), nest (33 degrees C), or intrauterine (36 degrees C) temperature. After 1-hr exposure to the experimental temperature, all pups were maintained at 33 degrees C and tested for their suckling response to an anesthetized dam. When newborns were tested at 120 min postpartum, simulated contractions increased the probability of nipple attachment in pups exposed to 21 degrees C relative to noncompressed littermates maintained at the same temperature. Atypically warm postpartum conditions (nestlike or intrauterine) obviated the effects of compression by increasing suckling above the levels seen in noncompressed newborns exposed to the cool condition. Thus, compressions facilitate the achievement of suckling under thermal conditions resembling those typically encountered by the newborn rat.