The breeding densities and breeding success of red grouse were compared with the performance of their main food plant, heather, at 17 study areas of 93-194 ha in north-east Scotland in 1959-61. Heather performance was assessed each spring and autumn by visual estimates which gave repeatable and accurate results when recorded by one observer. Breeding densities fluctuated from year to year, but on most areas the average density over several years was correlated with the quantity and age of heather. At 2 areas overlying base-rich rocks, the average density was exceptionally high in relation to the amount and age of heather. Heather samples from one of these areas contained more nitrogen, phosphorus, potassium, copper and cobalt than comparable samples from a nearby granite moor. Fluctuations in breeding densities from 1 spring to the next were not correlated with heather growth in the intervening summer. Consequently changes in breeding stocks were not due directly to quantitative variations in food production. Breeding success was correlated with heather growth and die-back in the preceding summer and winter. Thus breeding success was related to the amount of food available to breeding adults in winter and spring. Since changes in breeding density from one spring to the next were correlated with breeding success in the intervening summer, the size of the spring breeding stock was related to heather performance 12-21 mo. previously.