We determined the inhibitory effects of concentrations of tin- and zinc protoporphyrin (1-100 microM) and mesoporphyrin (0.1-10 microM) on the in vitro heme oxygenase (HO) (E.C.1.14.99.3) activity in liver, spleen, and intestine 13,000 x g tissue supernatants from fasted adult male Wistar rats through measurement of carbon monoxide by gas chromatography. All four metalloporphyrins inhibited intestinal HO, under the light-limited conditions of these experiments. The zinc porphyrins showed a clear concentration dependency over the entire range, reducing activity to near zero levels at their highest concentrations. The tin porphyrins reduced HO activity to 26% of initial levels, but the inhibition was not clearly concentration dependent. Liver and spleen supernatants exhibited concentration dependent inhibition by all four metalloporphyrins. We also assessed the effect of light on HO activity measurements. HO determinations in the light (8 microW/cm2/nm) yielded higher HO activity than for reactions performed in the dark. The presence of light and SnPP appeared to stimulate the HO activity of intestinal preparations thus overcoming the observed inhibition. Light and SnPP also decreased to a lesser degree the inhibition for the spleen preparation, but not for the liver. We conclude that heme oxygenase activity measurements via CO determination need to be conducted in the absence of light, in particular when photosensitizers are present. Furthermore, it appears that intestinal HO behaves in a quantitatively different way from other tissues, under varying conditions of metalloporphyrin inhibition and light exposure.