Data derived from mammalian, plant, and microbial models of genotoxicity may not be applicable to birds because of differences in avian genetic structure and physiology. The objective of this study was to develop a standardized, nonlethal genotoxicity assay for use with birds based on modification of a mammalian assay, flow cytometric measurement of variation in nuclear DNA content. Blood samples were collected from brachial veins of juvenile mallards (Anas platyrhynchos) before and after they were administered an oral dose of either methyl parathion (7.5, 15.0, or 30.0 mg/kg body weight), triethylenemelamine (0.25, 0.50, or 1.0 mg/kg body weight), or a solvent control. Cells were examined for nine parameters of DNA content and cell cycle kinetics. Results from blood samples were compared with results from spleen tissue, which is more commonly used in flow cytometric assays. Results were divided into three analysis groups: predose, postdose, and difference between pre-and postdose endpoints. Within triethylenemelamine dose groups, significant variation was found only in the predose postsynthetic gap (phase of cells after DNA synthesis; G 2 ) to presynthetic gap (phase of cells before DNA synthesis; G 1 ) ratio. Methyl parathion groups varied significantly in two parameters: postdose coefficient of variation of the G 2 peak and postdose G 2 to G 1 ratio. Dose levels of positive control groups may have been too low to elicit a definite genotoxic response. Despite the limited response in the positive control, evidence of disturbance of normal cell cycle kinetics suggests flow cytometry is a viable alternative for genotoxicity analyses in birds.