The influence of diet and maternal parity on the fatty acid composition of mature breast milk have been studied in 23 rural Gambian mothers. The subjects' habitual diet was low in fat (16% total energy), groundnuts (peanuts) constituting the principal fat source. The high abundance of oleic and linoleic acids in groundnut fat were reflected in the proportions of these fatty acids in breast milk (47.0 +/- 1.5 and 13.0 +/- 0.3 g/100 g fat, respectively), which were elevated compared with mothers in other African communities. Estimates of daily breast-milk fatty acid secretion indicated that there was little risk of essential fatty acid deficiency in Gambian infants. The proportions of fatty acids synthesized de novo (10:0, 12:0, 14:0) were less than expected from published studies of mothers consuming low-fat diets, averaging 16.8 +/- 1.4 g/100 g fat. As the study was conducted at a time of food shortage and high energy expenditure, it is argued that mobilisation of body fat during negative energy balance increased the availability of long-chain fatty acids to the breast. The proportion of endogenous fatty acids was markedly reduced in the milk of mothers of very high parity (parity 1 = 19.3 +/- 1.6 g/100 g fat; parities 10 + = 11.4 +/- 1.5 g/100 g fat; p less than 0.01). It is hypothesised that this represents an impairment of the ability to synthesise breast-milk fatty acids de novo in these mothers.