  We consider the probability distribution of large deviations in the
spin-glass free energy for the Sherrington-Kirkpatrick mean field model, i.e.
the exponentially small probability of finding a system with intensive free
energy smaller than the most likely one. This result is obtained by computing
$\Phi(n,T)=T \bar{Z^n}/ n$, i.e. the average value of the partition function to
the power $n$ as a function of $n$. We study in full details the phase diagram
of $\Phi(n,T)$ in the $(n,T)$ plane computing in particular the stability of
the replica-symmetric solution. At low temperatures we compute $\Phi(n,T)$ in
series of $n$ and $\tau=T_c-T$ at high orders using the standard hierarchical
ansatz and confirm earlier findings on the $O(n^5)$ scaling. We prove that the
$O(n^5)$ scaling is valid at all orders and obtain an exact expression for the
coefficient in term of the function $q(x)$. Resumming the series we obtain the
large deviations probability at all temperatures. At zero temperature the
analytical prediction displays a remarkable quantitative agreement with the
numerical data. A similar computation for the simpler spherical model is also
performed and the connection between large and small deviations is discussed.
