The role of the pulmonary alveolar macrophages (PAM) in the lung defense mechanism was evaluated in horses infected with equine hespesvirus-1 (EHV-1). Five adult horses were exposed to 10(6.6) TCID50 EHV-1 by intranasal instillation. Cytology of bronchoalveolar lavage (BAL) was performed using cytocentrifugation of samples and slides stained by Rosenfeld. Cell concentration was adjusted to 2´10(6) cells/ml, for the measurement of macrophage activity - spreading, phagocytosis of zymosan particles and release of hydrogen peroxide (H2O2). All animals were positive in virus isolation on the second, third and fifth days post-inoculation (DPI). Seroconversion was observed on the 14th DPI. Lymphocytosis was observed by BAL cytology on the 16th DPI. Measurement of macrophage activity demonstrated a marked increase in the spreading rate, on the 23rd and 30th DPI. Phagocytosis was decreased on the second DPI, and returned to levels similar to those observed before inoculation on the 23rd DPI. The amount of H2O2 released by PAM declined on day 2, but, by day 16, they returned to values similar to those observed before inoculation. The decline in PAM activity in the acute phase of disease is indirect evidence that these cells have an important role in lung defense mechanisms against this agent.