Trials at Tulloch, Aberdeen (sandy loam soil, 820 mm rainfall) and Woodside, Elgin (light sandy loam, 730 mm) compared organically managed crop rotations containing different proportions of spring oats, swedes, potatoes and grass/clover leys (0·50 and 0·67 of the rotation at Tulloch; 0·38 and 0·50 at Woodside). The trials simulated farm conditions through the use of grazing animals and the recycling of farmyard manure. The rotations at each site gave similar financial outputs. Yields of oats were higher where these were grown after the main ley phase of the rotation than where they were grown later in the rotation (more ears/m2 and grains/ear), but were not significantly higher after a 4-year ley than after a 3-year ley at Tulloch. It was concluded that all of the rotations were agronomically and financially sustainable. Cereal yields showed large year-to-year variations but little indication of a progressive decline. There were only small changes in soil organic matter, soil P and soil K. Increased early summer weed cover in the arable crops was not matched by increases in weed invasion in the grass/clover leys and did not appear to be affecting yields.