Rose tissue containing cytidine deaminase converts cytidine to uridine and ammonia gas. Rose tissue sensor was constructed by immobilizing 50mg of a rose petal tissue on an NH3 gas sensor and the optimum condition of the sensor for the determination of cytidine was investigated. The tissue sensor showed a linear range of$7.0 { imes} 10^{-4}$∼$1.0{ imes} 10^{-2}$M cytidine with a slope of 53 mV/decade in 0.2 M phosphate buffer, pH 8.4 at 37$^{circ}C$. The detection limits were $3.0{ imes}10^{-4}$ M and relative standard deviation was 3.4%. This sensor showed an excellent selectivity among various nucleosides and amino acids.