Ammonium chloride was added to diets varying in Ca content to evaluate its potential in preventing silica urolith formation in sheep. A 2 x 2 factorial experiment involved wether lambs with ad libitum access to a diet of 50% grass hay and 50% ground oats plus supplement. The basal diet contained on a DM basis 3.3% SiO2, .31% Ca, .22% P, 11.6% CP, and 26% ADF. Treatments (38 to 39 lambs/treatment) consisted of a control (C), limestone to increase dietary calcium to .6% (L), 1% ammonium chloride (A), and L + A (LA). After a 118-d experimental period, siliceous kidney deposits were found only in C and L, with silica making up 93% to 95% of the urolithic ash. Urolith incidences were 13% (C) and 18% (L), respectively. The lack of urolith development in lambs fed A and LA (ammonium chloride effect, P less than .01) and a trend toward a lower urolith incidence in C vs L (P less than .02) support the hypothesis that acid-forming effects of the diet and a reduction in the dietary Ca to P ratio reduce silica urolith formation.