Superoxide formation and lysosomal enzyme liberation circulating neutrophils has been assessed in healthy subjects. Blood samples were obtained at 8:00, 14:00, and 20:00 o'clock. The formation of superoxide followed a circadian rythm. Lowest concentration was found a 14:00 hours (4.59 +/- 2.09 nM reduced cytochrome/10(6) neutrophils). No 24 hour variation of betaglucuronidase and lysozyme liberation was proven. The number of neutrophil receptors for formyl-methyonil-leucyl-phenyl-alamine (FMLP) at the same hours did not show any circadian variation. The circadian changes of the functional capacity of the neutrophils might be one of the mechanisms that could justify the 24 hour variation of symptomatology characteristic of some diseases.