The production at central rapidity of K⁰_S, Λ, Ξ and Ω particles in Pb-Pb collisions at 158 A GeV/c has been measured by the NA57 experiment over a centrality range corresponding to the most central 53% of the inelastic Pb-Pb cross section. In this paper we present the rapidity distribution of each particle in the central rapidity unit as a function of the event centrality. The distributions are analysed based on hydrodynamical models of the collisions.