A supplement system for the control of equine nutritional secondary hyperparathyroidism (NSH) was evaluated on 4 farms in the Arcadia Valley of the Queensland brigalow region. Thirty-three local stock horses (of which 13 had clinical NSH and 7 were recent introductions) were grazed on buffel grass (Cenchrus ciliaris) for the 6 months from September 1979 to February 1980. Each horse was fed 1.0 kg of a mixture of ground limestone plus dicalcium phosphate (1:2) in 1.5 kg molasses each week. The pasture was hazardous during this time (total oxalate content above 0.5% and calcium: oxalate ratio below 0.5), but no new NSH cases occurred and those horses with clinical NSH improved, most becoming normal. After the first 6 months of supplementation, 3 new cases occurred on one farm. Doubling of the supplement dose and substituting rock phosphate in molasses for the previous mixture corrected the problem. The breakdown was thought to be partly because the supplement dose was slightly inadequate and partly because of behavioural factors.