This study was undertaken to characterize the different phases of the allergic pleurisy induced by ovalbumin in actively sensitized rats. The reaction was triggered by the intrathoracic injection of ovalbumin (12 micrograms/cavity) into animals sensitized 14 days before. The challenge caused, at 30 min, a drastic mast cell degranulation and exudation which peaked within 4 h. At this time, an intense pleural leucocyte recruitment also occurred, accounted for by an increase in the mononuclear cell counts and by a predominant influx of neutrophils. After 24 h, the mast cell counts started to recover, accompanied by a long-lasting (96 h) accumulation of pleural eosinophils. Forty-eight hours later, the exudation and neutrophils were at basal levels, whereas mast cell counts increased progressively to reach control values at 120 h. This study describes the time course of the exudative and cellular alterations observed during pleural inflammation induced by low antigen concentrations.