Fatty acid composition and oil formation were followed in developing seeds of two "linolenic acid lines" of low erucic acid rapeseed (Brassica napus L.) containing above and below normal linolenic acid levels to determine how these different levels of linolenic acid are derived. Under controlled growth room conditions, patterns of fresh weight and dry weight accumulation showed that seeds of both lines matured over the same period of time. During this time, both lines accumulated equivalent amounts of oil. The fatty acid composition of the oils of mature seed of both lines were similar for palmitic (16:0), palmitoleic (16:1), stearic (18:0), linoleic (18:2) and eicosenoic acid (20:1) when expressed either on a percent basis or a weight basis. However, oleic (18:1) and linolenic (18:3) acid were reciprocally related on a percent basis. The seeds of the two lines accumulated different weights of oleic and linolenic acid. Since the accumulation occurred over the same length of time, the difference in oleic and linolenic acid content was caused by different rates of accumulation. Equivalent chlorophyll contents, obtained for both lines, failed to show a relationship between chlorophyll content (indicative of photosynthetic activity) and linolenic acid accumulation.