The effects on the fertility of three commercial dairy herds of three types of copper- and seleniumcontaining mineral supplements was investigated. As the cows on each farm were dried off they were allocated to one of three treatment groups, and treated with either subcutaneous injections of copper and selenium, or two matrix intraruminal trace element boluses, or two glass intraruminal trace element boluses. When the data from the 406 cows on the three farms were combined, there was a significant difference between the conception rates of the three groups (P<0-001). The cows treated with the glass boluses conceived at a rate 1-8 times greater than those treated by injection (P<0-001), and at a rate 1-5 times greater than those treated with matrix boluses (P=0-002). These differences were associated with a significantly higher likelihood of service resulting in a conception in the group treated with glass boluses than in the group treated by injection (P=0-004). After adjusting for time from calving, time from treatment, time of year and farm, there was a significant (P=0-012) difference in glutathione peroxidase activities between the treatments, with the group treated by injection having a significantly lower activity than the groups treated with boluses.