Isabgol (Plantago ovata Forsk), is an annual herb cultivated as a medicinal plant in recent decades. Its seed contains mucilage, fatty oil, large quantities of albuminous matter, a pharmacologically inactive glucoside, namely Aucubin (C 13 H 19 O 8 H 2 O), and a plantiose sugar. Medicinal plants are rich in secondary metabolites: their biosynthesis is controlled genetically and is affected strongly by environmental factors. In the present work, the influence of sowing dates (20 th April, 5 th

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... d 20 th May) and nitrogen-fertilizer levels (0, 50, 100 and 150 kg ha-1) was examined on the growth, seed yield and seed swelling factor (content of mucilages) of isabgol. The statistical design was a split-plot arrangement of a randomized complete block design with three replicates: the sowing dates and nitrogen fertilizer were considered as the main and sub-main plots, respectively. The plot experiment was installed in the experimental station of Zanjan region located in the northwest of Iran, under silty-clay soil conditions. According to the resulting data, the best time for sowing isabgol in such a region was 5 th May and the suitable amount of nitrogen fertilizer was 100 kg ha-1. To discover any relationship, all measured independent variables were muliple regressed on the dependent variable (seed yield) followed by stepwise regression analysis. This verified that the seed yield among other independent characteristics tested appeared to show a significant positive effect on the seed swelling.