The objective of the present microcalorimetric investigation on mucosal biopsies from human gastric body (B) and descending duodenum (D) was to test whether a steady state of metabolism does persist and, if not, to find out how to assess the preexisting in vivo conditions. The measurements were performed at various incubation temperatures and partial pressures of oxygen. The measured curves fell exponentially after reaching an initial maximum without an initial steady state. Back-extrapolations to the time of taking the biopsies on the basis of a two-phase exponential decay curve gave baseline thermal power values at 37 degrees C of 41 (D) and 15 (B) mu W/mg(dw), respectively, which roughly correspond to known values of basal oxygen consumption of heart and kidney. The higher values of the duodenum may be founded on a higher transport activity in vivo. Altogether, microcalorimetry seems to be a method for comparative evaluation of metabolic activity of human mucosa samples.