In 2 experiments, rats were given a mixture containing high doses of 1-tri-iodothyronine and 1-thyroxine-sodium on Day 4 after birth (Group T). Controls (C) received equivalent amounts of saline. During the preweaning period, half of the litters in the T and C groups were raised in an impoverished (I) condition, and half in an enriched (E) condition. Open-field ambulation, rearing, defecation, and, in Experiment 2, grooming behavior was recorded during a 4-day period at 210 days (Experiment 1) and at 90 days of age (Experiment 2). A consistent augmenting action of neonatal hormone stimulation on ambulation and rearing was found for both males and females. Hormone-treated animals did not habituate their rate of general motor activity with repeated testings in the open-field. An over-all pattern of a larger effect of hormone-treatment in the I condition than in the E condition was noted, particularly among the females. Thus, the experience of preweaning enrichment partly "normalized" the open-field behavior of the T groups. The results are discussed in relation to an hypothesis concerning the interaction of rate of development of the central nervous system and environmental stimulation in determining later behavior.