Pregnancy rates after frozen semen inseminations (AI), particularly in older and problem mares, are lower than after fresh semen AI. Uterine contractility and the inflammatory reaction after frozen semen insemination were studied in two groups of mares: the abnormal group comprised of 6 old barren mares categorized in biopsy category IIB or III, and the control group including 6 reproductively normal young maiden mares in biopsy category I or IIA. All 12 mares were inseminated in the first cycle with 2 mL of phosphate-buffered saline (PBS) and in their second cycle with 2 mL of frozen semen containing 800x10(6) spermatozoa. Before and 1, 2, 4, 8, and 20 to 24 h after this treatment, all mares were examined by ultrasonography for intrauterine fluid accumulations (IUFA). The examinations were videotaped to count the number of uterine contractions later. Uterine fluid was obtained by tampon before treatment, and by the tampon method followed by uterine lavage after the last examination. Fluids were cultured bacteriologically, and polymorphonuclear leukocytes (PMN) were counted. Trypsin-inhibitor capacity (TIC), lysozyme concentration, and P-glucuronidase (BGase) and N-acetyl-beta-D-glucosaminidase (NAGase) activities were determined in frozen-thawed tampon and lavage fluids. Both treatments induced significant neutrophilia in the uterine lumen. Although PMN concentrations were numerically higher after frozen semen AI than after PBS-treatment, the difference was not significant. There was not any difference between the mare groups either. The amount of IUFA differed only in the normal group between frozen semen AI and PBS treatment, and between 0- and 24-h samples for frozen semen AI. Although abnormal mares showed consistently more fluid than normal mares, this difference was not significant. Uterine contractions and enzyme concentrations between groups did not differ. None of the variables showed significant differences between the normal and abnormal mares in their reaction to frozen semen AI.