Thioproline (a proline analog) at 10 mM induced proline (PRO) accumulation in oil palm polyembryogenic (PE) cultures. However, this treatment eventually killed the cultures, presumably due to some degree of toxicity. PE cultures utilized the exogenous proline (ExoPro) and ornithine (ExoOrn) more efficiently in liquid medium than in solid medium. The Pro accumulating cultures which were treated with ExoPro, ExoOrn, or subjected to low temperature stress returned to normal levels on transfer to normal media and conditions. These reversible changes in cellular Pro concentration are consistent with Pro being readily utilizable and so it can be considered a labile metabolize in oil palm PE cultures.