The immunosuppressive and anti-inflammatory properties of progesterone (P) have been established. The authors investigated whether the intraperitoneal instillation of P would lessen postoperative adhesion formation in New Zealand white rabbits undergoing pelvic surgical procedures. In phase I, severe, peritoneal lesions were made in the right uterine horn (n = 48). Animals were randomized to receive equal volumes of either (1) Ringer's lactate (RL); (2) 32% dextran 70 (HY; Hyskon Division, Pharmacia, Piscataway, NJ); (3) 500 mg P in oil (PO); or (4) 500 mg aqueous P (PA) at initial laparotomy. In phase II, the distal right uterine horn, including the mesosalpinx, was excised and microsurgical anastomosis was accomplished (n = 45). Aqueous P was not used in phase II; otherwise, the same agents were tested. Six weeks later, the severity of the adhesions formed was graded. The mean adhesion scores for the RL and HY groups were low for the right side in both phases and did not differ (P greater than 0.05). In contrast, higher scores were observed in all the P groups, regardless of the P preparation used or the surgical procedure performed (P less than 0.05).