The present investigation is focused on the two-dimensional, transient behavior of convective heat transfer in porous, rectangular ducts saturated with a fluid and in which there is uniform internal heat generation. In earlier works the steady-state multiplicity features of this flow have been studied. In the present work the evolutionary path to such steady states is examined. In several cases, a sustained oscillatory behavior has been observed. The solution structure is governed by two parameters, namely the aspect ratio of the duct,γ = b/a and the Rayleigh number, R = KβgaA′Qgαvk. For a duct with an aspect ratio of unity, a complicated solution structure is observed upon increasing the dynamical parameter. A steady, symmetric two-cell pattern observed for R of up to 4400 gives way to a periodic regime for R of up to 5400, then to a chaotic regime over a narrow range of R and a return to a steady-state solution at R = 5800. Upon increasing γ to 8, several multiple steady-state solutions are observed. The transition to oscillatory convection occurs at an earlier value of R with increasing y. None of the oscillatory solutions are symmetric about the center line.