Euthanasia efficacy and plasma biochemical implications of intraperitoneal thiamylal sodium in three rat strains

Acceptable euthanasia methods must rapidly induce unconsciousness and death without pain or distress. We evaluated intraperitoneal thiamylal sodium (TM; 150 mg/kg; hereafter TM150) in male Sprague-Dawley, Wistar, and Fischer 344 rats. TM produced immobilization, loss of righting reflex, respiratory arrest, and cardiac arrest in all strains, with the shortest latencies in Fischer 344. Plasma chemistry showed strain-dependent shifts, including aspartate aminotransferase (AST) increases and variable alanine aminotransferase (ALT) responses; chloride and calcium showed small, non-uniform changes across strains, and most values remained within commonly cited reference ranges. These findings most likely reflect perimortem physiology and methodological factors superimposed on strain baselines rather than primary organ injury. TM150 is therefore effective for rat euthanasia, although post-TM plasma biochemistry should be interpreted cautiously.