  The vertical structure of stationary thin accretion discs is calculated from
the energy balance equation with heat generation due to microscopic ion
viscosity {\eta} and electron heat conductivity {\kappa}, both depending on
temperature. In the optically thin discs it is found that for the heat
conductivity increasing with temperature, the vertical temperature gradient
exceeds the adiabatic value at some height, suggesting convective instability
in the upper disc layer. There is a critical Prandtl number, Pr = 4/9, above
which a Keplerian disc become fully convective. The vertical density
distribution of optically thin laminar accretion discs as found from the
hydrostatic equilibrium equation cannot be generally described by a polytrope
but in the case of constant viscosity and heat conductivity. In the optically
thick discs with radiation heat transfer, the vertical disc structure is found
to be convectively stable for both absorption dominated and scattering
dominated opacities, unless a very steep dependence of the viscosity
coefficient on temperature is assumed. A polytropic-like structure in this case
is found for Thomson scattering dominated opacity.
