An analytical investigation of the P-delta effects in medium height steel moment resisting frames excited by selected earthquake motions is reported. The frames with various design drifts and fundamental natural periods of free vibration were designed according to the loadings provisions for New Zealand's highest seismic risk zone (Wellington) of the Draft Code of Practice for General Structural Design and Design Loading for Buildings, DZ4203 [10] as issued as a "Draft for Comment" in May 1986. The effect of strength degradation was investigated. Dynamic magnification factors for the member forces and displacement were also observed. Limits for the maximum plastic hinge rotation, inelastic drift, displacement magnification and prediction of maximum plastic hinge rotation based on the inter storey drift are suggested.