In this paper we give optimal lower bounds for the blow-up rate of theḢ s T 3 -norm, 1 2 < s < 5 2 , of a putative singular solution of the Navier-Stokes equations, and we also present an elementary proof for a lower bound on blow-up rate of the Sobolev norms of possible singular solutions to the Euler equations when s > 5 2 .