High dietary levels of magnesium (greater than 2 g/kg drymatter [DM]) have been implicated as the main causal factor of urinary calculi in concentrate-fed lambs. Experiments were performed to try to reproduce this effect of high magnesium (from added calcined magnesite) on the incidence of urolithiasis in lambs. In the first, store lambs were given diets containing magnesium in the following concentrations: A,2.9; B,8.6 g/km DM. Twenty-four blackface lambs were each allocated to A and B and the control group of 160 lambs was fed a combined dried grass/sugar beet pulp pellet and hay ad libitum (diet C). The animals were housed and fed ad libitum until they were killed at an average liveweight of 35 kg. No animals showed clinical signs of urolithiasis and post mortem only a trace of calculous material was recovered from one lamb fed diet A. In the second experiment housed Suffolk and Suffolk cross lambs were fed a starter beginning at three to four weeks old and changed to a finisher at a liveweight of 23 kg. The feeds were either high magnesium starter (5.7) and finisher (6.0) or low magnesium starter (2.3) and finisher (2.8 g/kg DM). Twenty-two single lambs and 32 twins were assigned to both high and low magnesium diets. Lambs fed additional magnesium drank more water/kg DM intake and one of the single lambs showed clinical signs of urolithiasis and was killed. No others showed clinical signs and from the 62 killed there was no post mortem recovery of calculous material.