A computer-based decision support tool (p-EMA) has been developed to support UK Government policy of optimising agricultural pesticide use. The system estimates risks to a wide range of taxonomic groups and environmental compartments using methods consistent with current regulatory assessments, but also allows adjustments to reflect formulation, the local conditions and the environmental costs and benefits of varying management practices. Simple models of the dispersion pathways of the pesticide in the local environment are used to estimate predicted environmental concentrations in the field and margin soil, surface water and groundwater. Exposure estimates are then combined with the toxicological properties of the pesticide in the form of toxicity:exposure ratios. Concentrations in groundwater are calculated on the basis of a meta-version of the MACRO model linked to environmental and pesticide databases. Surface water concentrations are taken as the maximum of those arising from inputs via spray drift and drainflow.