  We compute the O(alpha_t alpha_s) two-loop corrections to the neutral Higgs
boson masses in the Minimal Supersymmetric extension of the Standard Model. An
appropriate use of the effective potential allows us to obtain simple
analytical formulae, valid for arbitrary values of mA and of the mass
parameters in the stop sector. We elucidate some subtleties of the effective
potential calculation, and find full agreement with the numerical output of the
existing diagrammatic calculation. We discuss in detail the limit of heavy
gluino.
