In the present study heat production rate (P) of rat kidney tissue in vitro has been measured with a microcalorimetric technique. Cortex slices (0.3-0.7 mm) were incubated in a balanced electrolyte solution or in medium E-199. The metabolic activities in these different media were found to be 0.87 microW/mg and 0.98 microW/mg, respectively, determined by using 5-mg slices after 1 h of incubation (P1h). Medulla slices with a thickness of 0.4 mm demonstrated a lower P, i.e., 0.65 microW/mg. The heat output was not influenced by the thickness of those slices, ranging from 0.3 to 0.7 mm. However, an increase in the wet weight of the tissue sample was accompanied by a reduced P, expressed as power per milligram wet weight. The magnitude of this crowding phenomenon decreased as the time of incubation increased. Storage of tissue for 3.5 h prior to measurement caused a 6-7% reduction of P. This study indicates that the microcalorimetric technique is a sensitive method for measuring metabolism in small kidney biopsies.