The blood flow of the sinus mucosa was studied in 11 New Zealand White rabbits. Their body weight averaged 3.5 kg. The blood flow determinations were performed during neurolept anaesthesia with continuous blood pressure recording. The blood flow in the sinus mucosa and septum, as well as in other organs, was determined both with Sn113-labelled microspheres and with Rb86Cl. Cardiac output as determined with the microsphere method was 146 +/- 36 ml X min-1 X 1000 g-1 body weight. There was a good correlation between the blood flow values obtained with the two methods. The blood flow in the sinus mucosa was 0.58 +/- 0.41 ml X min-1 X g-1 when measured with microspheres and 0.43 +/- 0.34 ml X min-1 X g-1 with the Rb86Cl technique. In the septal mucosa the corresponding values were 0.42 +/- 0.20 ml X min-1 X g-1 and 0.49 +/- 0.43 ml X min-1 X g-1 respectively.