  The aim of this paper is threefold. First, we obtain the precise bounds for
the heat kernel on isotropic Heisenberg groups by using well-known results in
the three dimensional case. Second, we study the asymptotic estimates at
infinity for the heat kernel on nonisotropic Heisenberg groups. As a
consequence, we give uniform upper and lower estimates of the heat kernel, and
complete its short-time behavior obtained by Beals-Gaveau-Greiner. Third, we
complete the results obtained in \cite{Li12} about the heat kernel of Grushin
operators.
