Twenty-five apparently healthy adults (13 men and 12 women), mean age 29.5 years (SD 3.6 years), served as subjects in a 24-h study conducted in Barcelona (Spain) in spring of 1990. Six blood samples were collected from each subject at 4-h intervals over a 24-h period, beginning at 1000. The oral temperature was measured at 2-h intervals to facilitate an independent biological time reference. The serum concentrations of apolipoprotein (apo) A-I, apo B, cholesterol, high- and low-density lipoprotein cholesterol, and triglyceride were measured. The circadian rhythms of these quantities were studied with use of a periodic function resulting from the sum of two cosine functions of 24 and 12 h. For serum concentrations of apo A-I and apo B, only the 24- or 12-h components, respectively, were significant. However, the other quantities studied were significantly affected by both rhythmic components. The maximum daily rhythmic variation ranged from 5% (with respect to the daily mean) for apo A-I to 63% for triglyceride concentration. In all cases except triglyceride, the daily maximum values were measured in the afternoon samples, near the time of maximum oral temperature.