Abstract Two feeding trials were carried out with lactating cows during early spring (September) and late spring (November). Three groups of animals were used; one received a starch supplement at each milking, the second a peanut oil supplement at each milking, and the third (control) received no supplement. Feed during the experimental periods was ‘Grasslands Ruanui’ perennial ryegrass, with a mineral composition believed to be conducive to hypomagnesaemic tetany. In the first trial plasma magnesium levels of mature animals were markedly lowered on all treatments by feeding Ruanui ryegrass; the effect on young animals was much smaller. No cases of clinical tetany were observed. Depression of plasma magnesium levels was lessened by starch supplementation and increased by oil supplementation. Plasma magnesium and phosphorus levels were positively correlated for individual cow means over the experimental period. Plasma calcium levels lay in the normal physiological range throughout Faecal levels of higher fatty acids and fatty acid soaps closely correlated with depression of plasma magnesium levels in mature cows, but were less closely correlated in younger animals. Pasture levels of trans-aconitic and citric acids were in the normal range and were considered unimportant. In the second trial the effects of Ruanui ryegrass feeding and of the other treatments were consistent with the results in the first trial, but the differences were much smaller.