The toxic effects of melamine given directly or in the feed to merino wethers were studied. A single dose of 100 g increased urea in blood from 28 to 315 mg per 100 ml for a period of 11 days. There was complete loss of appetite and excretion of urine ceased on the tenth day. When the sheep was examined post mortem on the eleventh day the tubules of the kidney were packed with crystals. Nephrosis and erosive abomasitis were seen also. Daily doses of 50 and 25 g killed the sheep after 7 and 9 days, respectively. In those sheep the blood urea was high just before death and post mortem crystals in the kidney tubules, nephrosis, haemorrhagic cystitis and acute typhlitis were seen. The dose of 50 g also caused ulcers in the abomasum. With 10 g daily one sheep did not die but 2 did so after 16 and 31 days. The 2 sheep which died lost appetite and stopped urinating 3 days before death and urea and creatinine in blood then increased sharply. There were crystals in the kidneys and severe oedema of the lungs. In another experiment volume of urine was reduced by offering water freely for 1 h on alternate days. Melamine was mixed with maizemeal and offered to 3 sheep to supply 7 g per sheep per day. All sheep survived for 6 weeks and there was no excessively high value for urea in blood. When daily intake of water was restricted to 600 ml much of the maizemeal containing melamine was refused but the sheep showed no ill effect. The production of white crystals on cooling, the appearance of a white deposit on the addition of picric acid and absorbancy of acidified crystals at 235 m micro indicated the presence of melamine in the urine of treated sheep. Melamine had no effect on pH of contents or motility of rumen and there was no sign of damage to liver.