In 75 clinically normal unspayed female control dogs between two and eleven years old the average plasma level of aspartate aminotransferase (AST) was 21.6 +/- 5.7 (+/- SD) IU/l, of alanine aminotransferase (ALT) 40.4 +/- 13.0 IU/l and of the AST/ALT ratio 0.6 +/- 0.2. These values showed only minor changes over years. In 96 bitches with the cystic hyperplasia-pyometra complex there was a very significant increase of the AST, decrease of the ALT and increase of the AST/ALT ratio. The changes were more pronounced in 62 clinically ill bitches with typical endometritis post oestrum, in 18 dogs with gram negative organisms in the uterus and in 53 bitches with white blood cell (WBC) levels higher than 40 X 10(9)/1. Renal failure had no influence on the specific changed values. The changed values returned either temporarily to normal after prostaglandin (PGF2 alpha)-treatment or definitely after ovariohysterectomy.