The effect of physiological and pharmacologically induced thymus involution was studied in 12-week-old female C57BL mice. Thymus involution was estimated by measurement of the thymus weight and the ability of thymocytes to induce a graft-versus-host (GvH) reaction at 48 h after delivery or drug administration in comparison with control (virgin, saline-treated) mice. The thymus weight and immunoreactivity of thymocytes after delivery were reduced in a statistically significant manner by ca. 80 and 75%, respectively. On the other hand, hydrocortisone administration decreased the thymus weight (by ca. 60%), but did not change the ability of thymocytes to induce a GvH reaction. Cyclophosphamide administration significantly reduced both the thymus weight and the reactivity of thymocytes. The present study suggests that the transient thymus involution observed after delivery, connected with a loss of the ability of thymocytes to induce a GvH reaction, cannot be explained merely by elimination of a steroid-sensitive cortical cell population, since the GvH reactivity of thymocytes was preserved in hydrocortisone-treated mice.