The present study was designed to test the hypothesis that changes in plasma concentrations of hormones may be responsible for the exercise-induced macrophage phagocytic stimulation. The effect of 30-min incubation of macrophages with plasma from mice previously exposed to swimming until exhaustion (with or without previous training) was studied, and the results showed a similar stimulation of the phagocytic capacity (attachment and ingestion) of these cells to that found in previous studies after exercise. Also, changes in plasma concentration of both thyroxine (T4) and 3,3',5-triiodo-L-thyronine (T3) after exercise were measured, and their effect on phagocytic capacity after in vitro incubation with peritoneal macrophages was investigated. Results indicated that, after exercise, plasma concentrations of T3 and T4 increased. Incubation of peritoneal macrophages for 30 min with a concentration of T3 similar to that observed in the plasma immediately after exercise (1.5 ng/ml) induced no modifications in the phagocytic capacity. However, a physiological concentration of T4 after exercise (75 ng/ml) stimulated the phagocytic capacity of peritoneal macrophages. In addition, a 10,000-fold greater concentration of these thyroid hormones did not modify the macrophage function. It is concluded that physiological concentration of T4 may be a mediator of the stimulation of the phagocytic function in macrophages induced by exercise.