Septic peritonitis (SP) occurs in 6-35% of dogs following gastrointestinal (GI) surgery. Objective criteria to identify surgical site dehiscence are currently lacking. The objective of this study was to identify, in dogs with closed suction abdominal drain (CSAD), objective laboratory markers to predict SP after GI surgery. A secondary objective was to determine the peri-operative pattern of abdominal fluid and serum NT-proCNP concentrations from dogs with SP. Thirty-three client-owned dogs undergoing intestinal surgery with placement of a CSAD were enrolled. Abdominal fluid and peripheral blood were collected pre-operatively and then daily via the CSAD until its removal. Analysis of all samples included WBC count, PCV, total solids, glucose, lactate, electrolytes, pH, PO2 and PCO2. Abdominal fluid analysis also included cytology, bacterial culture, and fluid volume. Blood-to-abdominal fluid glucose (BFG) and lactate (BFL) differences, abdominal fluid-to-blood lactate ratio, and blood-to-fluid WBC and neutrophil ratios were determined daily. NT-proCNP concentrations were also measured from abdominal fluid and serum from the last 12 dogs enrolled with SP. Outcomes included uneventful recovery (UR) or development of post-operative SP (POSP). Uneventful recovery was noted in 30 dogs, while 3 dogs developed POSP. None of the blood and fluid laboratory variables predicted POSP requiring relaparotomy. On day 3, blood-to-fluid WBC (p< 0.001) and neutrophil (p< 0.001) ratios were significantly higher in the POSP group compared to the UR group. NT-proCNP was successfully measured in canine abdominal fluid using an ELISA kit previously validated for canine serum. In dogs with SP, pre-operative abdominal fluid NT-proCNP concentrations were lower than concurrent serum concentrations (p= 0.031). Post-operatively, abdominal fluid NT-proCNP concentrations remained lower than serum concentrations (p< 0.050), except on day 4. Diagnostic tests to identify POSP following intestinal surgery are lacking. In dogs managed with a CSAD, identification of degenerate neutrophils or intra-cellular bacteria from abdominal fluid should not be considered solely as indicative of POSP. Use of blood-to-fluid WBC and neutrophil ratios to identify POSP requires further investigation. In dogs with SP, low serum NT-proCNP concentrations cannot be explained by abdominal compartmentalization.