Concern has been expressed that repeated use of pesticides may be leading to accumulation of residues in soil and to damaging effects on the environment. A long-term experiment, known as the Chemical Reference Plots, was started in 1974 on a silty clay loam soil at Rothamsted in which plots received applications of up to five pesticides (aldicarb, benomyl, chlorfenvinphos, glyphosate and chlorotoluron or triadimefon), each plot receiving the same treatment annually for up to 20 years. Spring barley was grown each year, and its yield was taken as an indicator of soil fertility. The glyphosate and triadimefon were applied to the autumn stubble prior to ploughing from growing seasons 1980 and 1982 respectively, chlorotoluron was sprayed pre-emergence (1974 and 1976 only) and the other compounds were incorporated into the soil in spring immediately before sowing (1974-1993 inclusive). No deleterious effects on crop productivity were observed from these pesticide applications, and no differences could be found in microbial processes in soils sampled in April 1992 save for a small increase in the amount of microbial-biomass carbon in plots receiving aldicarb. No pesticide residues could be detected in soil taken in August 1994, 17 months after the last experimental treatment. In laboratory incubations using these same soil samples, the degradation of aldicarb residues was greatly enhanced in plots that had received aldicarb for 20 years, whereas degradation rates of benomyl, chlorfenvinphos and triadimefon residues were not influenced by the treatment history.