  This paper is concerned with derivations in algebras of (unbounded) operators
affiliated with a von Neumann algebra $\mathcal{M}$. Let $\mathcal{% A}$ be one
of the algebras of measurable operators, locally measurable operators or, $\tau
$-measurable operators. We present a complete description of von Neumann
algebras $\mathcal{M}$ of type $I$ in terms of their central projections such
that every derivation in $\mathcal{A}$ is inner. It is also shown that every
derivation in the algebra $LS(\mathcal{M})$ of all locally measurable operators
with respect to a properly infinite von Neumann algebra $\mathcal{M}$ vanishes
on the center of $LS(\mathcal{M})$.
