Plasma renin activity (pra) and plasma aldosterone concentration (pac) were assessed in 23 dogs, nine of which had asymptomatic dilated cardiomyopathy (dcm) (New York Heart Association [nyha] class 1), eight had symptomatic DCM (nyha class III) and six had severe congestive heart failure due to dcm (nyha class IV). None of the dogs had received any drugs before the study. pra was increased in dogs with dcm (nyha classes III and IV) (median values 3·8 and 30·8 ng ml^-1 hour^-1) compared with normal dogs (median 0·89 ng ml^-1 hour^-1). However, pra was only marginally increased in dogs with dcm nyha class I (P=0·06). The pac was also increased in dogs with dcm (nyha classes III and IV) (median values 123 and 600 pg ml^-1) compared with normal dogs (median 61 pg ml^-1). The PAC was not significantly increased in the early stage of dcm (nyha class I). It was concluded that in dogs with dcm, the activity of the renin-angiotensin system (ras) was increased in nyha functional classes III and IV, and that the increase was correlated with the severity of the disease. The fact that the ras tended to be activated in dogs with asymptomatic dcm supports the rationale of early therapeutic intervention with inhibitors of the angiotensin-converting enzyme (ace) as first line therapy in congestive heart failure.