Seasonal variations of total blood histamine were examined in twenty patients with seasonal allergic rhinitis (Japanese cedar pollinosis). In blood specimens taken in the pollen season of 1978, the symptom-free season of 1978. and the pollen season of 1979, the mean values of blood histamine were 61.6, 50.0 and 84.2 ng/ml respectively. The differences in blood histamine levels between the pollen season of 1978 and the symptom-free season of 1978 and between the symptom-free season of 1978 and the pollen season of 1979 were statistically significant. In the symptom-free season of 1978, the blood histamine levels of twenty normal subjects were also examined: the mean value was 58.2 ng/ml. The blood histamine levels of the patients in the symptom-free season were not statistically different from those of the normal subjects.