The Monte Carlo simulation method is used to study the effects of nonuniformity on adsorption in slit-like pores. Three types of nonuniformity are considered : polydispersion of pore size diameters in the system, and geometrical as well as energetical heterogeneities of the pore walls. The differences and similarities in the pore filling in different systems are discussed. It is demonstrated that from measurements of the adsorption-desorption isotherms it is very difficult, if at all possible, to say anything about the origin of nonuniformity effects. On the other hand, the shape of the adsorption isotherms is, to some extent, sensitive to the distribution of nonuniformities in the system.