Background: Non-alcoholic steatohepatitis (NASH) is increasing in prevalence, yet the consequences for liver function are unknown. We studied ureagenesis, an essential metabolic liver function of importance for whole-body nitrogen homeostasis, in a rodent model of diet induced NASH. Methods: Rats were fed a high-fat, high-cholesterol diet for 4 and 16 weeks, resulting in early and advanced experimental NASH, respectively. We examined the urea cycle enzyme mRNAs in liver tissue, the hepatocyte urea cycle enzyme proteins and the in vivo Capacity of Urea-Nitrogen Synthesis (CUNS). Results: Early NASH decreased all the urea cycle mRNAs to an average of 60% and the ornithine transcarbamylase protein to 10% while the CUNS remained unchanged. Advanced NASH further decreased the carbamoyl phosphate synthetase protein to 63%, and in addition decreased the CUNS by 20% (from 5.65 ± 0.23 to 4.58 ± 0.30 μmol x (min x 100 g) -1; P = 0.01). Conclusion: Early NASH compromised the genes and enzyme proteins involved in ureagenesis, while advanced NASH resulted in a functional reduction in the capacity for ureagenesis. The pattern of urea cycle perturbations suggests a prevailing mitochondrial impairment by NASH. The decrease in CUNS has consequences for the ability of the body to adjust to changes in the requirements for nitrogen homeostasis e.g. at stressfull events. NASH, thus, in terms of metabolic consequences is not an innocuous lesion and the manifestations of the damage seem to be a continuum with increasing disease severity.