Decreased glutamine availability is proposed as a mechanism for changes in immune function with intense exhaustive exercise. Less is known about the immunomodulatory effects of regular nonexhaustive exercise. To determine the effects of low intensity regular exercise and dietary glutamine supplementation on plasma glutamine concentrations, lymphocyte metabolism, and immune function, male (278 +/- 5 g) and female (182 +/- 1 g) Sprague-Dawley Buffalo rats were fed nutritionally complete casein-based semi-purified diets +/- 2% w/w glutamine. Rats were trained (21 d), as confirmed by higher (P < 0.05) succinate dehydrogenase activity in soleus muscle, to swim 2 or 4 h.d-1 or remained sedentary. Exercise lowered plasma concentrations of tryptophan, glutamate, methionine, alanine, threonine, aspartate, asparagine, and ornithine and increased the lysine concentration (P < 0.05). Neither diet nor exercise altered plasma glutamine concentrations, lymphocyte phenotypes in spleen, or the in vitro rates of splenocyte energy metabolism (production of glucose and glutamine metabolites or ATP concentrations in the incubation media). Compared with nonsupplemented rats, splenic cytolytic activity (lysis of 51Cr labeled YAC-1 cells) was reduced (P < 0.05) in the glutamine-supplemented exercising group. Under these conditions, glutamine supplementation does not appear to provide any added benefit to the exercise-trained animal.