This study was designed to compare the efficacy of oral prednisolone and intramuscular (IM) dexamethasone in heaves-affected horses with environmental control. A total of 16 horses, aged 8-20years, with heaves were included in the study. Complete examinations were performed on Day 0 (before treatment), Day 13 (after treatment) and Day 30. Clinical variables, arterial blood gases, mucus scoring and carina evaluation (during endoscopy), and bronchoalveolar lavage (BAL) cytological analysis were all assessed. The horses were randomly assigned to receive either oral prednisolone (1mg/kg) or IM dexamethasone (0.1mg/kg). The animals were clinically scored and mucus accumulation evaluated. Results were analysed by repeated measures ANOVA with time (days of treatment) and treatment as the main effects. When combined with environmental control, prednisolone and dexamethasone treatments had similar effects on heaves score, blood gases and endoscopic scores. However, dexamethasone had a more beneficial effect on BAL cytology.