The single dose toxicity of 3-methoxy-6-allylthiopyridazine (K-6) was studied with Sprague-Dawley rats. K-6 was administered orally with dosages of 4.0, 3.0, 1.5, 1.0, 0.5, 0.25 and 0.1 g/kg to the rats. We observed daily the number of death, clinical signs, body weights and gross findings. All rats (6) were dead within a day after administration when doses of 4.0and 3.0 g/kg were administered. When dose of 2.0 g/kg was administered, 2 rats among 3 rats were dead. Any significant toxicity below 1.5g/kg of K6 was not observed when the different doses of 1.5, 1.0, 0.5, 0.25 and 0.1 g/kg were administered to 6 rats respectively.