This study was designed to determine the LD50 and phytochemical constituents of ethanol leaf-extract of Stachytarpheta cayennensis. Twenty-four mice were used for the acute toxicity (LD50) test. The mice were randomly assigned into seven groups of 1-7 with three mice in each group under the doses of 0, 100, 1000, 2000, 3000, 4000 and 5000 mg/kg body weight of the mice respectively. Group 1 served as the control. Groups 2-7 were administered with the extract with the corresponding doses and monitored closely for the periods of 72 hrs to ascertain the extract toxicity and/or lethal dose. Evaluation of phytochemicals composition and acute toxicity were known done using standard methods. The results revealed the following order of occurrence of phytochemicals (mg/100g): flavonoids > saponins > alkaloids > phenols > tannins > glycosides in the Stachytarpheta cayennensis ethanol leaf-extract. The acute toxicity result showed that even at extract dose of 5,000 mg/kg body weight of the mice, Stachytarpheta cayennensis ethanol leaf-extract did not cause death to any of the experimental mice, however, signs of toxicities were observed at the doses of 4000 and 5,000 mg/kg. This study buttresses the traditional uses of this plant in the management of some diseases conditions. Keywords: Phytochemical, Acute Toxicity, S. cayennensis, Ethanol and Leaf-extract