Let k be an algebraic closed field of characteristic zero. Let K be the
rational function field K=k(t). Let ϕ be a non isotrivial rational
function in K(z). We prove a bound for the cardinality of the set of
K--rational preperiodic points for ϕ in terms of the number of places of
bad reduction and the degree d of ϕ.Comment: There was a mistake in the previous version. The results have been
proven only for rational function field