Contrary to the common description of estrogen-induced thymus atrophy we have observed a thymus enlargement after treatment of neonatal female mice with estrogen. We now describe an age-dependent difference in the estrogen response (enlargement, atrophy) as well as mechanisms relevant to the response. Groups of female NMRI mice were treated with estrogen (diethylstilbestrol, DES) at different 5-day periods in prepubertal (day 1-5, day 6-10, day 30-34) or postpubertal life (days 48-52). All the treatment groups showed a reduced thymus weight 4 days after the last treatment but later responses differed. Neonatal DES treatment resulted in an ovary-independent thymus enlargement 8 weeks after the treatment when the cortical part was relatively larger than in controls; treatment on days 30-34 was followed by a rebound type of regeneration; the acute weight reduction after treatment on days 48-52 was normalized 16 days later. Neonatal DES treatment transiently depressed the number of thymic S phase cells 4 days after the treatment while apoptosis was similar in controls and DES females. The estrogen receptor pattern was not affected by DES. The number of white blood cells was temporarily depressed while the bone marrow cellularity was still reduced in 8-week-old females. Neonatal treatment with an LH-releasing hormone antagonist reduced thymus weight at 8 weeks but had no effect on the DES-induced enlargement. The delayed-type hypersensitivity response developed differently in controls and DES females. The thymus enlargement after neonatal estrogen treatment could be the result of an increased immigration of precursor cells into the thymus and/or a defect maturation/emigration mechanism. Further studies on different cell subsets are necessary to explain the mechanism behind the thymus enlargement.