An experiment was conducted for a period of 60 (sixty) days during March to May 2003 to study the effect of Rhizobium inoculum and N-fertilizer on bio-mass production in 3 (three) stages (40d, 50d and 60d) of maturity of cowpea plants following a randomized block design (RBD). There were six treatments as: To (uninoculated control), Ti (inoculated), T2 (25 kg N/ha), T3 (inoculated 25 kg N/ha), T4 (35 kg N/ha) and T5 (45 kg N/ha) having three replication in each treatment With the advancement of plant age, yield of cowpea forage on green as well as dry basis increased linearly. The maximum green biomass (265 q/ha) production was attained in treatment T5 at 60 days of plant age against 200 q/h in untreated control (To). Similar trend was observed in total dry matter and organic matter production (P<0.05) at harvest age of 60 days. Crude protein in different treatments raged from 5.39 to 4.74, 4.24 to 6.64 and 4.04 to 6.35 q/ha in 40, 50 and 60 days of plant age respectively. Maximum yield of crude protein attained at 50% flowering stage (50d) of maturity in all treatments. Rhizobium inoculum and/or nitrogen fertilizer application had significant (P<0.01) effect on green matter, DM, OM, and CP yield of cowpea forage. Rhizobium inoculum with or without N-fertilizer or nitrogen fertilizer alone significantly (P<0.01) and progressively increased plant height over the control but decreased leaf and stem ratio at all stages of maturity. Inoculum + N fertilizer and N-fertilizer alone with the application of 35 to 45 kg N/ha from urea showed a tendency to enhance rate of branching compared to control (P<0.05).