  We analyze renormalization and the high temperature expansion of the one-loop
effective action of the space-time noncommutative \phi^4 theory by using the
zeta function regularization in the imaginary time formalism (i.e., on S^1 x
R^3). Interestingly enough, there are no mixed (non-planar) contributions to
the counterterms as well as to the power-law high temperature asymptotics. We
also study the Wick rotation and formulate assumptions under which the real and
imaginary time formalisms are equivalent.
