A preliminary measurement of the Doppler effect in metallic thorium has been made in a fast neutron energy spectrum. The effect has been investigated up to slug temperatures of 500°C by an oscillator technique in which sensitivities on the order of 2 × 10-8 Δk/k can be attained with reasonable ease. The observed reactivity changes appear at this time to result largely from Doppler broadening of the resonances. This conclusion is supported by the fact that several, well-recognized correction terms have been found to be unimportant under the present circumstances. Based on a straight-line approximation to the measured points, the value of (1/ρ)(dρ/dT) was found to be 7.18 × 10-5/°C.