Paraquat was discovered in 1955 and introduced to the market place in 1962. During the 23 years between introduction and the present day numerous successful practical uses of the herbicide have been developed. In addition the characteristics of the chemical have allowed significant changes to be made in the ways that some crops are grown. Paraquat is a relatively non-selective foliage-applied contact herbicide. It is inactivated on contact with almost all naturally occurring soils and it was this property, perhaps above all others, that provided the greatest breakthrough in chemical weed control at the time of its discovery. Inactivation on contact with soil means that no biologically active residues remain in the soil, thus allowing planting or sowing to be carried out almost immediately after spraying. Although the non-systemic (contact) property of paraquat makes it less than ideal for the long-term control of perennial weeds, the same property is of real advantage when parts of crop plants are sprayed accidentally, for usually only the part receiving the spray is affected. Total annual usage of all herbicides in agriculture and horticulture in England and Wales, UK, over the period of 1980-1983 has been estimated at 26,360 tonnes used on 12402 X 10(3) ha (1 hectare = 1 X 10(4) m2). For paraquat (not including its mixtures with diquat and monolinuron) 270 tonnes were sprayed over 392,218 ha/year. It is estimated from sales records that in Europe 5 X 10(6) ha are sprayed annually with paraquat.(ABSTRACT TRUNCATED AT 250 WORDS)