This paper discusses some improvements on the first-order second-moment (FOSM) probabilistic approach to slope design. The stability model by Morgenstern and Price is used for the formulation of the performance function, thus enabling the FOSM method to be applied to the probabilistic assessment of a general slip surface. A new solution scheme is also used herein for Morgenstern and Price's method. It does not require iterations for the calculation of the interslice forces and the derivatives of the performance function can be evaluated analytically. The reliability index βHL defined by Hasofer and Lind is used as an index of safety measure. It has the advantage of being "invariant," that is, its value does not depend on the format of the performance function, a property considered lacking in the conventional reliability index. Reference is also made to the probabilistic modelling of soil profiles. The importance of the correlation structure of soil properties is highlighted and its effect on the reliability index βHL is discussed. Key words: slope stability, safety factors, reliability index, probability of failure, general slip surface, rigorous stability model.