The present work aims toward an approach to mountain pastures as a system of relationships in which it is needed to point out those environmental factors that reflect more clearly their structure and potential productivity. This initial step of pasture tipification is important as a previous activity before the planning and management of protected areas because these areas support an intensive pastoral activity. In order to achieve this objective a field study was conducted in two areas in the Gorbeia Natural Park (Bizkaia): Arraba and Aldamiñape, located at 900-1100 masl. In each pasture four plots were selected according to orientation (north/south) and slope (\u3c 10%/\u3e 10%), and botanical composition and potential yield determined, as well as the pasture rejected by the livestock at three different times (June-August-November). Besides the herbage biomass yield data were related to grass height. It is observed that the topographical factors studied do not have a clear effect on potential yield at the different areas; however, there is a clear seasonality, being august the period with the highest herbage biomass production. Analyzing the data of herbage rejected by the livestock it is noted a differential livestock distribution on the pasture through the grazing time, being the area of Aldamiñape facing north and with \u3e 10% slope the area with the highest grazing pressure, which in turn is the area with a highest cover degree by Agrostis curtisii, an species of low nutritional value
