Describes a study of 146 sheep farmers in Cumbria, Devon and north Wales, and 143 quarry worker controls, investigating the occurrence of chronic, acute and delayed health effects in relation to repeated exposure to organophosphates(OP's) through sheep-dipping. Cognitive functioning and psychiatric state were assessed in a cross-sectional design at least two months after exposure, whilst mood and symptom reporting were assessed prospectively, before, 24 hours after, and 3-4 weeks after exposure. Current exposure status was verified at testing 24 hours and two months after exposure by urine analysis, and lifetime exposure was assessed by questionnaire. An attitude rating scale was developed to quantify the perception of risk relating to OP sheep-dips. Exposure at dipping appeared to be confirmed although there did not appear to be any evidence of associated acute or delayed effects. However, in the absence of recent exposure, decrements were apparent amongst the sheep-dippers in tests of vigilance, and information and linguistic processing. For one test the decrement appeared to be dose-related. Sheepdippers also appeared more vulnerable to developing psychiatric disorder. Memory and psychomotor functions were, however, similar across groups. Inter-relationships between health effects, attitude and exposure characteristics were explored.