This study compares serum concentrations of tryptophan (TRP), kynurenine (KYN), kynurenic acid (KYNA) and indoleamine 2,3-dioxygenase (IDO) activity in healthy bitches and bitches with bacterial uterine infection (pyometra). The effects of surgery were also assessed by measuring these variables in both groups of dogs before and after ovariohysterectomy. Presurgery mean (±SD) TRP, KYN, and KYNA concentrations and IDO activity were 68.44 ±21.77 μmol/L, 2.00 ±0.33 μmol/L, 112.11 ±111.91 μmol/L and 29.22 ±10.10, respectively, in the healthy dogs; and 40.16 ±12.11 μmol/L, 8.27 ±3.94 μmol/L, 411.11 ±199.60 μmol/L and 205.92, respectively, in the dogs with pyometra. TRP and KYN levels had normalized upon suture removal (10 days after surgery) though IDO activity and KYNA concentrations remained elevated during the postoperative period compared to presurgery values in both study groups. Our results suggest that KYNA concentrations and IDO activity could be useful indicators of the inflammation induced by pyometra and could be also used to monitor recovery following ovariohysterectomy in both healthy dogs and dogs with pyometra.