This paper illustrates the use of deterministic and probabilistic approaches to the ecotoxicological risk assessment of effluents through two simple examples taken from the UK Direct Toxicity Assessment Demonstration Programme. In this study, the direct protection objective for the River Esk in Scotland was the prevention of short-term lethal toxicity to invertebrates from exposure to effluent containing insecticides. Both a deterministic and a probabilistic risk characterisation show that such toxicity is very unlikely to occur. The protection objectives for the Lower Tees Estuary were more complex, due to a variety of discharges and potentially sensitive species. A deterministic risk assessment using the most sensitive species suggests that toxicity will be high at all times. In contrast, a probabilistic risk assessment suggests that, although acute toxicity is considerable, it is not likely to occur at all times or affect all species, and discharge scenarios do exist in which acute toxicity could be relatively low. These two case studies illustrate the use of probabilistic risk assessment in achieving more realistic assessments of effluent toxicity through use of all available data.