Experimental evidence on forage yield stability of summer forage crops and their mixtures with legumes, in Egypt, is sparse. This study was carried out to examine the response and stability of seven summer forages; i.e, pearl millet "Penissetum glaucum L.", hybrid sorghum 102 "Sorghum bicolor L.", sweet sorghum "Sorghum bicolor L.", fodder cowpea "Vigna unguiculata L." and three grass-cowpea mixtures. Twelve trials were conducted from 2003 to 2005 in the Agricultural Experimental Farm of Alexandria University. All experiments were identical in design and treatments except for nitrogen fertilizer levels. A randomized complete block design with six replications was used to test the differences among the seven summer forages in each experiment. Forages differed for their regression on environmental index. Significant deviations from linearity of response were recorded for green and dry forage yields of hybrid sorghum 102-cowpea mixture and for dry forage yield of cowpea. Positive b_i values were obtained for green and dry forage yields, indicating that the studied forage crops might preferably be grown under a favorable environment; i.e, high nitrogen rates. Hybrid sorghum 102 and its mixture with cowpea were the most responsive forage to changes in environment, whereas, cowpea was the least. Hybrid sorghum 102, millet-cowpea mixture and hybrid sorghum 102-cowpea mixture were suggested to be grown under high nitrogen levels, since it expressed moderate or high values of S²_d and high levels of response (b_i). Cowpea that had the least rate of response and S²_d values might be proposed to favor the low fertility environments. Medium fertility environments might be better to suite the remaining studied forage crops.