This thesis examines human ammonia (NH3) metabolism in trained and untrained individuals. Specifically, these aspects include: 1) plasma NH3 accumulation during exercise at similar absolute and relative work intensities; 2) plasma NH3 accumulation during recovery from maximal exercise; 3) the absolute and relative work intensities at which blood lactate (LA) and plasma NH3 concentrations begin to rapidly accumulate (i.e the LA breakpoint - LABP, and NH3 breakpoint - NH3BP); 4) the relationship between the NH3BP and LABP, and 5) the relationship between plasma NH3, blood (LA), and muscle fibre type.