Abstract. Montgomery's pair correlation conjecture predicts the asymptotic behavior of the function N (T, β) defined to be the number of pairs γ and γ of ordinates of nontrivial zeros of the Riemann zetafunction satisfying 0 < γ, γ ≤ T and 0 < γ − γ ≤ 2πβ/ log T as T → ∞. In this paper, assuming the Riemann hypothesis, we prove upper and lower bounds for N (T, β), for all β > 0, using Montgomery's formula and some extremal functions of exponential type. These functions are optimal in the sense that they majorize and minorize the characteristic function of the interval [−β, β] in a way to minimize the L 1 R, 1 − sin πx πx 2 dx -error. We give a complete solution for this extremal problem using the framework of reproducing kernel Hilbert spaces of entire functions. This extends previous work by P. X. Gallagher [19] in 1985, where the case β ∈ 1 2 N was considered using non-extremal majorants and minorants.