two types of thick film biosensor with different substrate materials were tested for the preparation of conductivity sensors for the determination of urea. urea is hydrolyzed by urease to ammonia and carbon dioxide resulting in an increase in conductivity urease was immobilized by cross-linking with glutaraldehyde. the response times for both sensor fabrications were approximately 10 min. the first type of urea sensor was fabricated by printing two electrodes on conventional thick film al/sub 2/o/sub 3/ substrate using a platinum paste. the second type of thick film urea sensor was fabricated using 'green tape' technology which resulted in multi-layer strips with four electrodes printed with a silver palladium paste. the conductivity devices were characterized by admittance measurements. the admittance and the phase angle were monitored varying the amplitude of the ac voltage from 100 mv to 2 v peak-to-peak and the frequency from 0.5 to 50 khz. the influence of electrolyte concentration was also investigated in the range from 1 mmol/1 to 1 mol/1 kcl