(1) Certain aspects of leukocyte dynamics in rats were studied by means of a mechanical cross-circulation technic.
(2) Cross-circulation between two normal rats caused little change in the leukocyte counts.
(3) When irradiated leukopenic animals were cross-circulated with normals, the leukocyte counts of the normal animals were moderately decreased, while the level of the irradiated animals rose. Prolonged mixing of the circulations, however, failed to bring the counts of the two animals into equilibrium.
(4) After cessation of cross-circulation, the leukocyte counts of the normal donors usually rose slowly, while those of the irradiated animals fell progressively with a half-time disappearance of 30-60 minutes.
(5) The possible significance of these observations in terms of normal leukocyte mobilization, viability and destruction was discussed.