A study of age and sex matched groups of adult female sheep from 88 flat, hill, and high-country farms was conducted in the South Island of New Zealand to investigate the influence of breed and certain environmental factors on the prevalence rate of small-intestinal adenocarcinoma (SIA). 20 678 female sheep aged 5.5-7.5 years were examined at slaughter, 125 cases of SIA were found (6 per thousand) in animals from 61 farms (69%) and the prevalence rate for individual farm groups varied from 0 to 38 per thousand. Differences in tumour rate between breed groups were significant but differences between farm type were not. Exposure to phenoxy (Ph), picolinic acid (Pi) herbicides, or both (PhPi) was associated with significant increases in tumour rate. The increase in rate was significant for exposure to each of the 3 herbicide groups. Exposure to recently sprayed feed stuffs was associated with a significantly larger increase in tumour rate than exposure to less recently sprayed food. There was no difference between tumour rates of sheep exposed to Ph herbicides with or without 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Rates rose significantly with the total number of Ph, Pi, PhPi sprays used on the farm. The variation in rates associated with herbicides is sufficient to explain the breed differences recorded.