Brassicaceae seedmeals, by-products of mustard and canola oil production, average 6% nitrogen (N) by weight and have the potential for use as soil amendments to improve nutrient availability. Brassicaceae seedmeals contain different types and concentrations of glucosinolates (GLSs) that produce biologically active and often toxic compounds. Field and laboratory studies were initiated to investigate the influence of Brassica juncea, Brassica napus, and Sinapis alba seedmeals on N mineralization. Apparent N mineralization (ANM) during the growing season was 2 to 5 times greater in seedmeal-amended (2 t ha^-1) soils than in the untreated controls in field studies conducted in 2005 and 2006. For both years, seasonal ANM in the B. juncea and B. napus treatments was equivalent to ~70% of the total N applied in the seedmeal. High-GLS B. juncea and S. alba seedmeals resulted in significantly lower microbial biomass N than in the low-GLS B. napus treatment at 4 and 8 d after seedmeal application. In the laboratory study, ¹⁵N-labelled seedmeals were amended to soil at a 2 % (wt. meal:wt. soil) rate and incubated aerobically at constant temperature and moisture content for 45 d. Microbial respiration was lower and delayed in the high-GLS B. juncea and S. alba seedmeals compared to the low-GLS B. napus seedmeal. Mineralization of seedmeal N was initially rapid with 40 to 46% of seedmeal-¹⁵N recovered in the soil inorganic N pool by day 15. Nitrification appeared to be inhibited in the S. alba treatment in the laboratory study. Brassicaceae seedmeals substantially increased plant-available inorganic N in soil and may serve as an effective organic source of mineralizable N. Although it was of short duration, evidence of the biological activity of GLS-degradation products was observed in decreased microbial biomass nitrogen, soil respiration, and nitrification in high-GLS seedmeals.