Pendimethalin's herbicidal action lies in its inhibition of the steps in plant cell division responsible for chromosome separation and cell wall formation. Terrestrial studies show that 10-20% of the herbicide evaporates during the first weeks after application. The remainder may dissipate biologically or chemically. Half-maximal dissipation time, or half-life (DT(50)) values vary from a few days to >200 days. Field and laboratory studies showed that reduced temperatures and drought prolong dissipation time to as long as 72-2094 days. In freshwater, pendimethalin concentrations reach 6 microg/L after runoff. In water the 10% lethal concentration (LC(10)) for Daphnia was 6 microg/L when exposed via suspended food. The LC(50) was 78 microg/L. An invertebrate field study showed that soil nematodes were reduced by 35-60% at pendimethalin application rates of 0.75 and 1.0 kg/ha, respectively. After application, soil microbiota is affected by pendimethalin for approximately 4 weeks. Plant-Rhizobium symbiosis is affected at application rates of 0.5-1.0 kg/ha.