The distribution of the pyrimidinone glucopyranosides vicine and convicine was investigated in resting seed and axenic developing seedlings up to 17 days of age in Viciafaba cv Troy, a low seed pyrimidinone glucopyranoside mutant (MTG5) derived from Troy, and a related species V narbonensis. The lower amounts of total vicine and convicine in resting seed of MTGS were due primarily to a reduction in cotyledon pyrimidinone glucopyranosides, with the amount in the embryo axis much less affected. Distribution within developing seedlings revealed consistent patterns. A pulse of vicine appeared in both species in stems and roots at day 4 to day 7, then declined. Amounts of convicine rose initially then declined in seedling stems of both species, whereas in roots the rise in convicine continued throughout the period. The rise in convicine in the roots of V narbonensis could not have been accounted for by conversion from reserves of vicine. The low pyrimidinone glucopyranoside mutant MTGS had less vicine initially in stems and roots than both Troy and V . narbonensis but by 17 days this had risen close to the levels of Troy and exceeded those of V narbonensis. By flowering, amounts of vicine and convicine in glass-house grown plants were similar in MTGS and Troy. The levels of both compounds in the roots of MTGS exceeded those of the sown seed. Consequently, both V f a b a and V narbonensis have the capacity to synthesise vicine and convicine in young plants. A mutation in a gene controlling levels of pyrimidinone glucopyranosides in the developing seed of Vfaba does not suppress the biosynthetic ability of young plants.