Experiments were undertaken to examine the comparative toxicity to sheep of ammonium sulphate and urea given as single doses and the value of ammonium sulphate fed alone or in combination with urea as a supplement to low nitrogen roughages. Ammonium sulphate dosing produced significantly lower mean blood ammonia levels than urea dosing. Fifteen grams of nitrogen as ammonium sulphate were tolerated as a single dose but the equivalent level of urea was toxic. lnappetance and ruminal stasis was noted when sheep fed a low nitrogen roughage (0.5 per cent nitrogen) received ammonium sulphate supplement as the sole non-protein nitrogen source. Ruminal stasis and inappetance may have been due to sulphide toxicity. Ammonium sulphate when used at lower levels in varying combinations with urea was well utilized as a nitrogen and sulphur source even when supplying 50 per cent of the supplemented nitrogen.