Rice straw (RS) was treated with urea, ammonium bicarbonate (NH₄HCO₃) and sodium hydroxide (NaOH), respectively. Treatments with urea, NH₄HCO₃ and NaOH increased the in sacco dry matter degradability of RS from 45.3 (untreated) to 52.5, 53.2 and 63.6%, respectively (P<0.01). The untreated and treated RS stem samples were digested in the rumen of a Huzhou sheep for 12, 24, 48 and 72 h and examined for the degradation of tissues and cell walls by using a scanning electron microscope (SEM), and a transmission electron microscope (TEM). Parenchyma was slightly distorted by treatment with urea or NH₄HCO₃, but severely distorted by NaOH treatment. The chemical treatments had little effect on other tissues of RS stem. All three treatments accelerated the degradation of parenchyma at 12 and 24 h, and NaOH treatment made the parenchyma degraded to most extent. Sclerenchyma cell wall in the untreated RS was slightly digested and still left partial secondary wall and complete middle lamella and primary wall even at 72 h, but NaOH-treated sclerenchyma cell wall began to be degraded at 48 h and just left the middle lamella and occasional primary wall at 72 h. All three treatments made the phloem absent at 48 h, but had little effect on the digestion of epidermis and xylem of vascular tissue. These results indicated that NaOH treatment had the best effects on the modification of structure of rice straw stems and the digestion of tissues and cell wall, and that the positive effects of NH₄HCO₃ treatment basically paralleled those of urea treatment. The improvement in digestion of histological structures was accordant entirely with the increase of in sacco degradability of RS after the three treatments. It is suggested that the histological methods by means of microscopical techniques such as SEM and TEM are conducive to a direct insight into the mechanism with which chemical treatments exerted the effects on the improvement of nutritive value of RS.