Soybean (Glycine max L.) wasr grown in 2003 and 2004 to determine the effect of deficit irrigation given at five different reproductive stages on its growth, yield and seed composition. All soybean plots received full irrigation during the vegetative stage, while treatments. were started during one of the reproductive stages (R1-2, beginning of flowering and full bloom; R-3, beginning of pod; R-4, full pod; R-5, beginning of seed; R-6, full seed and full irrigation) by completely stopping irrigation. In 2003 and 2004, the observed leaf area index ranged from 3.9 (R-6) to 2.6 (control), and from 4.0 (R-6) to 2.7 (control), respectively. Water stress resulted in reduced vegetative growth, leaf relative water content and leaf chlorophyll content. In 2003 and 2004 seed yields ranged from 1955 (R-6) to 3684 kg ha(-1) (control), and from 1867 (R-6) to 3952 kg ha(-1) (control), respectively. Any water stress imposed on soybean plants in R-3, R-5 and R-6 resulted in substantial yield reduction compared with the full irrigated control treatment. There were no significant effects of total chlorophyll and leaf relative water content on the oil and protein content of soybean seed. Water stress during the different reproductive stages (R-1-R-6) significantly affected protein content, C18:0, C18:2 and C18:3 fatty acids. The lowest protein value was obtained in R1-2, while the highest values were obtained from R-4 in both years. Water stress effect imposed on any of the reproductive stages showed a significant effect on protein content but not seed oil. The highest protein content values were obtained from R-4 treatment at 42.5% and 42.1% in 2003 and 2004, respectively, while the lowest values were from R-6 and R1-2 at 37.9% and 38.9% in 2003 and 2004, respectively. Average seed oil content in 2003 and 2004 were 14.61% and 16.12%, respectively.