The 24 hr volume flow, cell concentration, and total cell output of thoracic duct fluid from mice with pertussis-induced hyperlymphocytosis were markedly reduced when compared with values obtained in normal animals. An increase in the number of circulating lymphocytes occurred in several of the pertussis-treated mice despite the presence of an indwelling thoracic duct cannula. The drainage from such animals also showed a reduced cell concentration and total cell output. It is suggested that lymphocyte recirculation may be minimal in pertussis-induced lymphocytosis, and the evidence obtained also suggests that lymphocytes may enter the blood stream by direct routes during the course of the reaction.