Alveolar macroph::ges (AM) play an important role in the regulation oflocal imrnune activity in the lung through production of bioactive factors in response to inflammatory processes and hypoxia. In the present study, the impact of hypoxia was examined by determining pulmonary AM and leucocyte numbers in lung lavage samples from 10 male Holsteins (aged 18-30 months) reared at high altitude and 9 reared at sea level. While total leukocytecounts forthe high altitude (216.62±106.221µ1) and sea level (117.09±15.71/ µl) animals were not statístically different, mean AM values were (p<0.02) with greater values for the former 202.37±91.53/µl (95.5±2.5%) than for the latter group 75.75±17.84/µl (65.11±11.62). Conversely, the numberoflymphocytes was significantly higher (p<0.0004) at sea level 10.92±4.19/µl (8.83±2.63%) than at altitude (0.62±0.35%), as were the values for polymorphonuclear cells (PMN) (p<0.0008) at 25.42±10.83% for sea level and 3.67±2.28% at hígh altítude. No differences in the frequency of basophils and eosinophils were found (p>0.05) at the two elevations. The increased numbers of alveolar AM at high elevation may possibly be a source of cytokines responsible for changes associated with chronic hypoxia at pulmonary level.