1. Guanine deaminase in rat brain and liver was distributed among all the subcellular fractions: nuclei, `heavy' mitochondria, `light' mitochondria, microsomes and the supernatant fluid. The greater part of the activity passed into the soluble fraction. Among the particulate components, the `light' mitochondria constituted the richest fraction. 2. The sum of the enzymic activities of the component fractions obtained on differential centrifugation was considerably greater than the activity of guanine deaminase in the whole homogenate. 3. The `heavy'-mitochondrial fraction had a powerful inhibitory effect on the guanine-deaminase activity of the supernatant fraction. 4. All the sedimented fractions, except the microsomes, gave rise to higher guanine-deaminase activity on treatment with Triton X-100. 5. The inhibitory capacity of the `heavy' mitochondria increased on treatment with Triton X-100; the detergent-treated nuclear fraction also brought about inhibition of the 5000g supernatant. 6. Guanine-deaminase inhibitor from the `heavy' mitochondria was solubilized by high-speed grinding of the particles, followed by treatment with Triton X-100. The inhibitor appeared to be protein in nature, since it was precipitated by trichloroacetic acid and by half-saturation with ammonium sulphate, and was non-diffusible. It was inactivated by heating at 50° for 5min. 7. It is possible that the guanine deaminase associated with particles differs from the soluble enzyme in its response to inhibitor.