Electron paramagnetic resonance studies have been carried out on two species of cytochrome P-450 ( P-450 scc and P-450 11β) purified from bovine adrenocortical mitochondria. The g values of the steroid-bound cytochromes in the high spin form were determined at 4.2° K to be 8.07, 3.60 and 1.70 for P-450 scc and 8.00, 3.65 and 1.71 for P-450 11β. The E/D values were estimated to be 0.103 for P-450 scc and 0.099 for P-450 11β. Either high spin P-450 was converted into the low spin form by the treatment with an NADPH dependent electron donating system and subsequent gel filtration in order to remove the steroid. The g values of the low spin ferric cytochromes were 2.423, 2.247 and 1.914 for P-450 scc and 2.430, 2.251 and 1.919 for P-450 11β at 77° K. The values for |Δ/λ|, | V/λ| and k were 5.69, 5.21 and 1.11 for P-450 scc and 5.94, 5.38 and 1.16 for P-450 11β. These studies indicate that there are some differences in the ferric heme environment between P-450 scc and P-450 11β.