Entropy and temperature of black 3-branes.

We consider slightly non-extremal black 3-branes of type IIB supergravity and show that their Bekenstein-Hawking entropy agrees with the counting of states of the Dirichlet 3-brane. The Dirichlet brane excitations are described in terms of the statistical mechanics of a 3+1 dimensional gas of massless open string states. This is essentially the classic problem of black body radiation. The black body temperature is shown to be identical with the temperature of the Hawking radiation. We also construct a solution of type IIB supergravity describing a 3-brane with a nite density of longitudinal momentum. For extremal momentum-carrying 3-branes the horizon area vanishes. This is in agreement with the fact that the BPS entropy of the momentum-carrying Dirichlet 3-branes is not an extensive quantity.