  We study the performance of QCD simulations with dynamical Wilson fermions by
combining the Hybrid Monte Carlo algorithm with parallel tempering on $10^4$
and $12^4$ lattices. In order to compare tempered with standard simulations,
covariance matrices between sub-ensembles have to be formulated and evaluated
using the general properties of autocorrelations of the parallel tempering
algorithm. We find that rendering the hopping parameter $\kappa$ dynamical does
not lead to an essential improvement. We point out possible reasons for this
observation and discuss more suitable ways of applying parallel tempering to
QCD.
