A transmitter has access to X, a relay has access to Y , and a receiver has access to Z and wants to compute a given function f(X; Y;Z). How many bits must be transmitted from the transmitter to the relay and from the relay to the receiver so that the latter can reliably recover f(X; Y;Z)? The main result is an inner bound to the rate region of this problem which is tight when (X,Y,Z) forms a Markov chain.