It is necessary to estimate extreme sea loads when probabilistic and reliability methods are used to analyse t~e ultimate strength of ship corresponding to the modes of failure due to yielding or inelastic buckling. The ship response records taken in full-scale tests show that the vertical stress time his tory consists of a rapidly varying time history with random amplitude and frequency, oscillating about a mean value. The mean value itself is a weakly timedependent function. The rapidly varying part includes a low frequency component due to the motion of ship as a rigid body, and a high frequency one due to the impact of the ship as a flexible body on the water, Le. slamming ("springing" of the flexible ship excited by the energy present in the high frequency wave components is not considered in this paper).