High intakes of crude protein (CP), in particular rumen degradable protein (RDP), have been associated with reduced fertility in cattle. This reduction in fertility has been attributed to putative toxic effects of elevated systemic concentrations of ammonia and, or urea, the by-products of protein metabolism, on gametes and embryos. The objective of the studies reported here was to examine the effect of CP and fermentable carbohydrate intake on various blood metabolites and on fertility, in beef heifers. In the first experiment, 40 heifers were randomly assigned to either low (no urea) or high (240 g urea) RDP, grass silage based diets, supplemented with 1.5 or 3.0 kg dry matter (DM) of either rolled barley (BAR) or molassed sugar beet pulp (MSBP) or no carbohydrate (control). The high RDP diet increased plasma ammonia and urea concentrations. MSBP at 3 kg DM/day was more effective than BAR in reducing ammonia (P<0.05) and urea (P<0.001). In animals fed no urea, MSBP at 3 kg DM/day was also more effective than BAR in reducing systemic ammonia (P<0.001) and urea (P < 0.05). In the second experiment, 83 beef cross-bred heifers were randomly assigned over two replicates to one of 4 treatment groups in a 2 x 2 factorial design. Animals consuming high (85 kg N/ha; HN) or low (0 kg N/ha; LN) nitrogen fertilised pastures were supplemented with either 0 or 3 kg dry matter (DM) MSBP daily as follows: 1) HN only (n=21); 2) HN + 3kg MSBP, (n=22); 3) LN only (n=21); 4) LN + 3kg MSBP (n=19). The HN pastures had higher (P<0.001) CP and lower (P<0.001) water soluble carbohydrate concentrations than LN pastures. Systemic ammonia (P<0.05) and urea (P<0.001) were higher in heifers on the HN pasture. Embryo survival rate was high overall (80%) and was not affected by systemic ammonia or urea concentrations or by supplementation with MSBP. Systemic progesterone, insulin and glucose were not affected by treatment. High CP intake coupled with low fermentable carbohydrate, lead to elevated systemic ammonia and urea concentrations in heifers. However, no adverse effect on fertility was observed.