  The Cahn effect and the unintegrated unpolarized parton distribution function
$f_{1}^{q}(x,\mathbf{p}_{T})$ are studied in a covariant approach. The Cahn
effect is compared with some other effects due to the parton intrinsic motion.
The comparison suggests that the present understanding of parton transverse
momenta and intrinsic motion in general is still rather incomplete. The new
relation for $f_{1}^{q}(x,\mathbf{p}_{T})$ is obtained in the framework of the
covariant parton model from which a prediction for this distribution function
follows.
