In connection with the preparatory work for the second Swiss National Forest Inventory, a model is being constructed with the aim of screening sampling designs and procedures to be applied. Some basic distributions, such as the distribution of tree diameter within a stand, are being derived from plot data of the first inventory. This paper describes the construction of a simple system of diameter distribution in uniform high forest. The observed distributions are fitted by means of the Weibull distribution, which employs three parameters. Because of the many-storeyed structure of many of the stands, the distributions were irregular in form. Dividing them into one distribution for trees of the upper storey and one for those of lower storeys allowed fitting by means of the Weibull distribution. The data were grouped into classes of stem count and basal area. All plots belonging to a particular class were grouped together for the fitting of a Weibull curve using the method of maximum likelihood. With a multiple linear regression over all classes within one stage of development, using the Weibull parameters as dependent variables, it was possible to construct a simple model with the Weibull parameters as linear functions of stand density and basal area. Other stand or site features exerted no substantial effect on the characteristics of the distributions. The aim of the diameter distribution system is to elaborate diameter distributions for each forest type and stage of development and for any given combination of stand density (number of trees per ha) and basal area (m2/ha) in such a way that they reflect the character of the forest type concerned.