The effects of MSMA on the development of straighthead, yields of rice, and hormonal balance in rice plants were observed in a greenhouse study. MSMA was applied to soil at 0, 6, and 9 kg ai/ha. At 6 kg/ha some straighthead and a 45% yield reduction occurred. At 9 kg/ha straighthead was evident, and yield was reduced by 85%. Analyses were made for ethylene with gas chromatography and for indoleacetic acid (IAA) with high performance liquid chromatography. Concentrations of ethylene and total arsenic detected in the tissue from rice leaves grown in MSMA-treated soil were higher at every growth stage when compared to plants grown in untreated soil. Both treated and untreated plants showed marked increases of these compounds during the reproductive stage and a decrease prior to panicle emergence. Significant changes occurred in IAA levels with the growth stage of treated and untreated samples, but MSMA had no obvious effect on IAA levels at any of the stages examined in this study.