Three techniques have been compared in order to study pasture improvement in the Mediterranean mountains of north-eastern Portugal: spontaneous vegetation, spontaneous vegetation plus fertilization and sown pastures plus fertilization. This experiment was conducted in six different locations and under several environmental conditions. Seasonal and annual production, and botanical composition (grasses, legumes, other families and bare soil) are presented for the 1998-2000 period. The most fertilized pastures produced the highest yields. The differences between the three pasture improvement techniques were reduced throughout these years and fertilized spontaneous vegetation held an intermediate position. The proportion of legumes was highest in sown and fertilized pastures and it increased over the course of the years. These pastures also provided the highest and most regular yield throughout the year.