Children under 5-years old from a random sample of 40 families of the Serere people in Senegal were studied for two years, in 1981 and 1982. Two visits took place each year, one in the dry season, one in the rainy season. On each visit, height and weight were measured for all children. For children aged over two years, measures of biacromial and bi-iliocristal diameter, and biceps, triceps and subscapular skinfold thicknesses were also taken. Significant variation was found for all measures according to season. There were no significant differences between boys and girls. There were increases in the measures of biacromial and bi-iliocristal diameter with age. Decreases with age in sub-cutaneous fat levels were compensated by a slight increase in muscle mass. Both height and weight velocities show seasonal variation, but the changes are not synchronised. Weight velocity was very low during the 1981 rainy season, whilst height velocity at the same period was comparable with that of English children. During the 1982 dry season, weight velocity was similar to that for European children, whilst height velocity for Serere children less than 3 years old was substantially reduced. Mean levels for each age group, together with the difference between observed and expected values of Quetelet's index show the effect of the 1981 rainy season on all children. These data also reveal the existence of a critical period between 6 and 24 months of age when Serere infants are particularly vulnerable.