An investigation was made of the hinging mechanism in under- reinforced concrete beams subjected to static or dynamic loads. Two test series on simply supported beams with a 6-foot span length were conducted. In one series, 11 beams were subjected to two concentrated loads symmetrically placed 18 inches apart; the primary variable was the magnitude of the step load pulse (1.0 to 1.4 times the static yield load). In the other series, 15 beams were subjected to a concentrated load at midspan; the primary variables were the type of load (static or dynamic), the amount of tension reinforcement (p=0.9,1.3, and 2.0, p1/p=0.67), and the size of the transverse reinforcement (1/8- and 1/4-inch round bars and No. 3 deformed bars). Hinge development was similar in the statically and dynamically loaded beams and resulted from the formation and propagation of a yielded zone or zones in the tension reinforcement. Strain hardening of the tension reinforcement increased the static resistance above the yield value; the increase ranged from 5 to 60 percent. Although decreasing the amount of tension reinforcement increased the deflection of certain stages, the ultimate rotation capacity was not significantly affected. In addition, the size of the transverse reinforcement had a negligible effect on the ultimate rotation capacity.