The short term regulation of heat production in brown adipose tissue mitochondria (BAT) of acutely Mg-deficient rats was demonstrated by comparing several parameters of mitochondrial energization. Mg deficiency in vivo had absolutely no effect on the BAT uncoupling protein concentration (UCP) which was only modified by thermal conditions. The same high concentration was observed 10 d cold exposed control and Mg-deficient rats. Four days of warm re-exposure at thermal neutrality led to a moderate 26 per cent decrease with both diets which was not modified by cold stress for 1 h. Proton conductance. CmH+, and proton motive force, delta p, were calculated from membrane potential and respiration rate measurements. The same high level CmH+ was observed in cold exposed rats with both diets. Compared to warm re-exposed control rats, CmH+ was threefold higher in the corresponding Mg-deficient group which indicated a much lower masking of the proton channel of UCP with the Mg-deficient diet. This difference was not dependent on the presence of magnesium in vitro. The basal CmH+, independent of UCP, was not altered by magnesium deficiency. These results emphasize that acute regulation of thermogenic BAT activity through the masking and unmasking process is altered when magnesium supply is limited in vivo.