The investigation has been carried out on stimulated and unstimulated peritoneal cavities of rats. China ink and Corynebacterium parvum were injected i.p. both as peritoneal stimuli and markers. Omenta were picked up at time intervals beginning with 10 min and up to seven days after the i.p. injection. The light and electronmicroscopic investigation showed after 10-30 minutes labeled macrophages stuck as monolayers on some peritoneal areas corresponding to the milky spots which developed in size and number. Days after the i.p. injection the labeled macrophages were found deeper in the milky spots. After the fourth day they appeared in the regional lymph nodes. The milky spots contained also large lymphocytes and plasma cells. The results suggest that milky spots are not only places of resident macrophages development and release in the peritoneal cavity but also their exit pathways. Therefore the omentum leads the traffic of peritoneal macrophages. The developed milky spots play also the role of lymphoid structures providing grounds for macrophage-lymphocyte contacts.