Summary The lowering effect of carbon tetrachloride (CCl 4) on the amount of hepatic microsomal cytochrome P-450 of male rats was studied in vitro. It was found that CCl 4 added directly into the media containing microsomal suspensions decreased the amount of cytochrome P-450 detectable by difference spectra at zero time of incubation. The incubation of the media with CCl 4 at 37° in an open system or in a closed system did not decrease the content of cytochrome P-450. As cytochrome P-420 was not detected in the addition of CCl 4 lower than 10 μl, it was postulated that the decrease in the content of cytochrome P-450 was not due to its conversion to cytochrome P-420. The activity of NADPH-cytochrome c reductase was not affected by CCI 4. The results indicate that the decrease of the amount of detectable cytochrome P-450 by CCl 4 is due to the destruction of cytochrome P-450 and the binding of cytochrome P-450 with CCI 4 in a lower concentration of CCl 4 and in addition it is due to the conversion of cytochrome P-420 from cytochrome P-450 in a higher concentration of CCl 4.