Seasonal influences on circadian rhythms in the blood picture of SPF rats housed under artificial illumination.

Total leukocyte, erythrocyte, absolute lymphocyte, eosinophil and segmented neutrophil counts in the peripheral blood of SPF rats kept under light/dark 12 h/12 h cycles were investigated with respect to a time standardization of blood drawing for haematological examinations in long-term studies, especially in preclinical safety evaluations of new drugs. Mesor and amplitude were subject to seasonal variations in the circadian rhythm of leukocyte, absolute lymphocyte, segmented neutrophil and eosinophil counts. Variations in erythrocyte counts were sometimes statistically significant but they were moderate and no circadian rhythm was evident. Thus, examinations of the rat blood picture during biomedical studies have to be related to the pertinent time of circadian clocks and in long-term studies also to the time of season.