The insecticidal action of acephate has been related to its conversion to methamidophos (MP) through decarboxyamidase or to a combined anticholinesterase effect of acephate and MP. The ingestion of acephate was suspected in one case because a nearly empty bottle of it was discovered in a room of the house where the body was found. In the second case, the ingestion of acephate was suspected because an almost empty bottle of a liquid pesticide was found under the deceased's feet in the car where he died. He also had several cuts on his body, and a bloody single-edged knife was near the body. In both cases, when the autopsy started, more than 18 hours had elapsed from the finding of the corpse. Analyses of acephate and MP were performed by gas chromatography/mass spectrometry (GC-MS) on extracts obtained by the modified method of Maroni et al. This paper describes the procedure in detail. In the first case, the blood acephate was 149 mg/ml, and MP was 3.0 mg/ml. Serum pseudocholinesterase (ChE) activity was inhibited. No significant finding of injury or disease was determined to be the cause of death; acute poisoning by acephate was strongly suspected. In the second victim, the blood acephate was 46 mg/ml, and MP was not detected. ChE activity was in the normal range. Hemorrhage from the knife wounds was the suspected cause of death. The authors advise that the toxicity of acephate chemical is primarily due to MP. 2 tables and 18 references