Abstract: The presence of antinutrients and toxic substances in vegetables limits the derivable benefits from vegetables. The levels of these substances in vegetables are influenced by the nature of soil in which the vegetables are grown. The effect of applied nitrogen fertilizer on the levels of some antinutrients and toxic substances is investigated with a view to determine the appropriateness or otherwise of the application of nitrogen fertilizer in growing vegetables. Pot experiments were conducted to determine the effect of soil nitrogen levels on soluble and total oxalates, cyanide, nitrate and some micronutrients namely, vitamin C, β-carotene (precursor of vitamin A) and mineral elements (Fe, Mg, Zn, Cu, Ca, Na and K) in Hibiscus sabdariffa. The leaves of the vegetable were harvested and analysed at market maturity (vegetative phase) and fruiting (reproductive phase) of the plant development. Results obtained showed that the applied nitrogen fertilizer significantly elevated nitrate and β-carotene contents, while it decreases the levels of vitamin C, soluble and total oxalates in the vegetable. The levels of cyanide and mineral elements were not significantly affected by the applied nitrogen fertilizer.