Cyclic 7-8 wk old virgin albino rats were used. Bilateral ovariectomy was performed on 22 rats under ether anesthesia. Eleven of the animals were subjected to uterine trauma by sewing 3 cotton-thread (No. 50) loops into each uterine horn. Commencing 1 day after surgery, ovine prolactin (200 .mu.g in 0.1 ml 0.9% NaCl/injection) or 0.1 ml solvent was given s.c. once daily to all animals at 12:00 h for 6 consecutive days. Trauma induced marked hypertrophy of uterine tissue as indicated by increased wet and dry weights (P < 0.01). The uterine hypertrophy in the traumatized animals receiving prolactin was significantly greater (P < 0.01) than in those of the other groups. The uteri of the non-traumatized rats receiving prolactin did not show any significant change from those of controls. Thus trauma increased the uterine sensitivity to prolactin in the absence of the ovaries. Neither uterine ballooning nor growth of deciduomata was noted in any of the animals.