During mammalian pregnancy the maternal thymus undergoes significant involution, and then recovers in size after birth. The mechanism behind this involution is not known, but it has been suggested that elevated levels of hormones during pregnancy induce the involution. We have recently shown that injection of 17β-oestradiol into mice causes loss of early thymocyte precursors and inhibits proliferation of developing thymocytes. This suggests that elevated oestrogen in pregnancy may contribute to thymic involution. We have investigated this idea by examining the fate of thymocytes during mouse pregnancy in much greater detail than has been previously reported. Looking over a broad time–course, we find that pregnancy does not affect thymocyte precursor populations in the bone marrow, but induces a profound loss of early thymic progenitors in the thymus as early as day 12·5 of pregnancy. This loss is accompanied by decreased thymocyte proliferation, which returns to normal 2–4 days postpartum. No enhancement of apoptosis is detectable at any stage of pregnancy. We also find that there is a reduction in recent thymic emigrants after oestrogen treatment and at day 17·5 of pregnancy, suggesting that thymic involution during pregnancy influences the peripheral T-cell repertoire. The similarities between oestrogen-mediated involution and pregnancy-mediated involution suggest that oestrogen is a significant contributor to loss of thymocyte cellularity during pregnancy, and probably functions primarily by reducing thymocyte proliferation.