Behavior of rat fetuses following chemical or tactile stimulation.

The behavior of fetal rats was examined on Day 19 of gestation with procedures that enabled chemical stimulation and direct observation of fetuses. Rat fetuses are sensitive to both tactile stroking and intraoral infusion of chemical solutions, but the pattern and amount of activity depend upon the modality of stimulation. Fetal responsiveness is affected by prior experience with chemical stimuli. Repeated exposure within a 10-min period results in a waning of response, and repeated exposure across a delay of 48 hr results in a different pattern of response than is seen to a novel stimulus. Reexposure to a stimulus experienced earlier in gestation also alters fetal responsiveness to other forms of tactile and chemical stimulation. These findings indicate that the rat fetus exhibits olfactory function in utero and suggest central processing of sensory information, including evidence of habituation, a fetal orienting reflex to novel stimuli, and the existence of prenatal behavioral states associated with different patterns of response.