The aim of the present study was twofold: (a) to confirm a previously observed negative relationship between plasma catecholamines and the percentage of natural killer (NK) cells in peripheral blood from resting human subjects, and (b) to examine the relationship between the size of the spleen and the composition of circulating lymphocyte subsets in resting subjects. A total of 14 young healthy male subjects were investigated in a supine resting position. Lymphocyte subset composition was determined with two-colour flow cytometry, and lymphocyte subsets were expressed as percentages of mononuclear cells. Spleen size was evaluated with ultrasonography. Plasma catecholamines were determined. Plasma norepinephrine and the percentage of NK-cells (CD3-CD56+) were negatively correlated (rs = -0.62, p = 0.019). The CD4/CD8 ratio and plasma norepinephrine were positively correlated (rs = 0.57, p = 0.037) and the major part of this correlation was due to a correlation between plasma norepinephrine and the percentage of CD4+ cells. The percentage of NK cells (CD3-CD56+) was predicted by a multiple regression model including the percentage of CD8+ cells and the spleen index, a measure of spleen size (r = 0.93, p < 0.001). The correlation in the resting state between plasma norepinephrine and the percentage of NK cells (negative correlation) on the one hand and the CD4/CD8 ratio (positive correlation) on the other contrasts with the acute mobilizing effects of epinephrine, isoproterenol and exercise on lymphocyte subsets. These relationships remain unexplained, but results accord with the hypothesis that catecholamines may have dual effects on lymphocyte subsets. The results support the view that the spleen may have a depot function for NK cells.