This study tested the hypothesis that increased processing or efficacy of the unconditioned stimulus (US) contributes to the facilitation of trace conditioning that occurs when preweanling rats are conditioned in a novel sensory-rich context. Experiment 1 extended previous findings (D. L. McKinzie & N. E. Spear, 1995) of facilitated acoustic trace conditioning in the 17-day-old rat in a sensory-enhanced versus a home odor context. In the enhanced or more familiar context, Experiment 2 tested rats of this age for degree of spontaneous locomotor activity and ultrasonic vocalizations, activity and ultrasounding in response to shock, and the acoustic startle reflex. The enhanced context resulted in a greater overall activity response to shock, increased ultrasounding to discrete shocks, and a sensitized latency to startle. The results suggest that enhanced US processing in a sensory-rich context is a likely contributor to its facilitative effect on infant learning.