With the objective of identifying the causes by which seed vigor differences affected plant performance, a study was conducted under laboratory conditions. Growth analyses were accomplished in seedlings of two black oat (Avena strigosa Schreb.) cultivars, with several seed vigor levels. Reductions in vigor caused reductions in seed soluble proteins levels. High vigor accelerated metabolic processes in seeds, leading to a quicker and uniform emission of radicules. Highly vigorous seeds produced seedlings with higher growth rates in the initial period after germination, resulting in larger initial seedling size. Genotipic differences were observed for some parameters evaluated in response to seed vigor. Root length showed to be a more reliable parameter than shoot length, to evaluate vigor differences among seed lots of cultivated black oats.