The effects of training are dependent on complex, adaptive changes which are 
induced by acute physical exercise at different levels. In particular, evidence 
shows that the hypothalamus-pituitary-adrenocortical axis, as well as the 
sympatho-adrenomedullary system, is mainly involved in mediating the 
physiological effects of physical exercise. The aim of the present study was to 
investigate, through a morphological and biochemical approach, the effects of 
training on the adrenal gland of mice, following two different protocols 
consisting of either low- or high-intensity training. Mice were run daily on a 
motorised treadmill for 8 weeks, at a velocity corresponding to 60% 
(low-intensity exercise) or 90% (high-intensity exercise) of the maximal running 
velocity previously determined by an incremental exercise test. We found that 
physical exercise produced an increase in the adrenal gland size compared with 
the control (sedentary) mice. The increase was 31.04% for mice that underwent 
high-intensity exercise and 10.08% for mice that underwent low intensity 
exercise, and this appeared to be the result of an increase in the area of both 
the adrenal cortex and adrenal medulla. Morphological analysis of the adrenal 
cortex showed that both types of exercise produced an increase in cytoplasmic 
vacuoles in steroidogenic cells, appearing more abundant after high-intensity 
exercise. No change was found in the reticulate zone. In the adrenal medulla, 
despite the absence of morphological changes, immunohistochemistry for tyrosine 
hydroxylase, dopamine β-hydroxylase and phenyl-ethanolamine-N-methyltransferase 
demonstrated an increased immunopositivity for these cathecolamine-synthesizing 
enzymes after intense exercise. These results were confirmed by immunoblot 
accompanied by densitometric analysis.