  We estimate the vertical gradient of rotational velocity for several spiral
galaxies in the framework of a global thin-disc model, using the approximation
of quasi-circular orbits. We obtain gradients having a broad range of values,
in agreement with measurements, for galaxies with both low and high gradients.
To model the gradient, it suffices to know the rotation curve only. We
illustrate, using the example of galaxy NGC 4302 with particularly high
gradients, that mass models of galactic rotation curves that assume a
significant spheroidal mass component reduce the predicted gradient value,
which may suggest that the mass distribution is dominated by a flattened
disc-like component. We conclude that the value and behaviour of the vertical
gradient in rotational velocity can be used to study the mass distribution in
spiral galaxies.
