  In this paper, we show that any solution of the nonlinear Schr{{\"o}}dinger
equation $iu\_t+\Delta u\pm|u|^\frac{4}{N}u=0,$ which blows up in finite time,
satisfies a mass concentration phenomena near the blow-up time. Our proof is
essentially based on the Bourgain's one~\cite{MR99f:35184}, which has
established this result in the bidimensional spatial case, and on a
generalization of Strichartz's inequality, where the bidimensional spatial case
was proved by Moyua, Vargas and Vega~\cite{MR1671214}. We also generalize to
higher dimensions the results in Keraani~\cite{MR2216444} and Merle and
Vega~\cite{MR1628235}.
