Wu Lin powder (WLP) is a prescription that causes urination to remove dampness. In order to elucidate its mechanism of action, the effect of WLP on atrial natriuretic factor (ANF) was observed. The results showed that the level of ANF was significantly higher in the plasma of mice after giving WLP as well as giving Alisma orientalis or Cinnamomum cassia orally, compared with control, P < 0.05, 0.01 and 0.001 respectively. The effect was most evident at 45 minute after giving WLP (P < 0.05). It is suggested that ANF might play an important role in therapeutic action of WLP.