  We present some details of our recent theoretical investigations on the total
cross section of the reactions $pp\to pp \pi^0$, $pp\to pp \eta$ and $pp\to
p\Lambda K^+$ at threshold. Applying standard nonrelativistic perturbation
theory within the framework of meson exchange models (MEC) to all of the
investigated processes we observe that theory is still far--off from a
quantitative understanding of the existing data, although the qualitative
behaviour of the threshold cross sections can be reproduced. The often quoted
$\eta^2$--dependence of the $pp\to pp \pi^0$ cross section at threshold is due
to a calculational mistake of Koltun and Reitan (1966) and not based on
theoretical grounds, where threebody phasespace behaviour would yield $\eta^4$,
which can be affected by final state and short range effects. To respect the
high momentum transfers for $pp\to pp \eta$ and $pp\to p\Lambda K^+$
relativistic models were developed. Hereby the $\eta$--production still is
based on MEC, while the $K^+$ production was investigated in the
quark-gluon-picture based on the Bethe--Salpeter formalism.
