Experimental acute sinusitis was induced in 21 New Zealand hybrid rabbits by occluding the ostium and inoculating them with Streptococcus pneumonia. While a group Of rabbits with sinusitis was left untreated, two other groups were administered parenteral sodium nitroprussid (SNP) and oral levofloxacin for ten days. While staphylococci species, non-hemolytic streptococcus and contaminated flora were isolated from the sinuses of controls, Streptococcus pneumonia was re-isolated in two of six untreated rabbits, in one of six SNP administered rabbits and none of the levofloxacin treated rabbits. Serum and maxillary sinus mucosal nitric oxide (NO) levels were correlated. While the mean maxillary sinus NO level of controls was significantly higher than that of untreated rabbits, the mean maxillary sinus and serum NO levels were significantly higher in SNP administered rabbits than in the others. Although goblet cell hyperplasia and squamous cell metaplasia were detected in some slides, edema and neutrophil infiltration were the prominent findings. The most severe inflammatory changes were found in the untreated sinusitis group on the third and fifth days. The earliest improvement was observed in the levofloxacin treated rabbits. It was concluded that NO level is decreased during acute sinusitis and that SNP administration hastens the bacteriological and histological recovery.