The increase in energy necessity due to intense exercise leads to a shift in the ATP/ADP ratio, with production of AMP. The metabolism of AMP increases ammonia production and subsequent raise in blood urate. Ammonia raises in blood is considered a possible mediator to central fatigue. Caffeine is a widely used ergogenic aid used in several sports by different athletes. Previous studies in our laboratory showed that caffeine supplementation raises ammonemia in athletes, an effect in amino acid metabolism probably explained by the decrease of the urea cycle enzymes activities. Here we describe the effects of caffeine supplementation on nitrogen metabolism in male Wistar rats submitted to a force training protocol. The animals were trained in a weight lifting machine using an electrostimulator to induce the desired movement. Rats received caffeine (5 mg.Kg^-1) and glucose or glucose alone as a control one hour before the exercise and were trained during 15 days (caffeine, C; caffeine with exercise, CEx; glucose, G and glucose with exercise, GEx). Our results showed that glucose supplementation decreased ~ 30% the total blood ammonia in the first training day, this effect was not achieved in the other groups (CEx, C and G). The training leads to a decrease in basal ammonemia in the groups supplemented with CEx (~20%) or GEx (~35%). This decrease was enhanced by exercise to 50 % in the CEx group and 65% to GEx. Blood urea was sickly lower in animal receiving glucose supplementation. We do not measure significant difference in the amounts of serum proteins; albumin; triacylglycerols; cholesterol; high density lipoproteins; creatinine.  Our data suggest a protective effect of training in ammonia increase induced by exercise, an effect that is raised by caffeine supplementation.