Since 1948 the International Whaling Commission has restricted the catch of whales from the Antarctic by an annual limitation on total catch. This restriction has made the decline of the blue and fin whale stocks slower than would have occurred with no regulation. Assuming that the regulation has affected the length of season, but not the number or composition of the expeditions, the probable effort in each season in the absence of regulation has been calculated. From the relation between stocks abundance and total effort since 1946 the differences in adult stock with and without regulation, and hence in recruits four years later, and ultimately in total catch, were calculated. It is estimated that the quota regulations have increased the total catch since 1946 by some 4,300 blue whales and 45,000 fin whales. Despite this comparative success the International Whaling Commission has failed in its primary task of maintaining a viable whaling industry in the Antarctic.