It is shown by tracer experiments with DL-[2-3H,5-14C]- and DL-[(RS)-5-3H,5-14C]rnithine, that the metabolic conversion of ornithine into proline, in three plant species (Nicotiana tabacum, Datura stramonium, and Lupinus angustifolius), takes place with maintenance of the delta-hydrogen atoms but with loss of the alpha-hydrogen atom. This indicates a route via alpha-keto-delta-aminovaleric acid (5-amino-2-oxopentanoic acid) and disproves the accepted route via glutamic gamma-semialdehyde (2-amino-5-oxopentanoic acid).