A micro-sized polyvinyl chloride (PVC) membrane Li+-selective electrode without inner filling solution was constructed. The emf measurements were made by using a micro-sized solid silver/silver chloride reference electrode. The potentiometric response characteristics of the electrode were examined under steady-state conditions. The developed electrode showed high selectivity for lithium with respect to common alkali and alkaline-earth metal cations. The working concentration range of the electrode was 1.0 x 10(-1) to 6.0 x 10(-5) mol L-1 with a slope of 53.6 mV decade(-1), and pH working range was found to be 6-10. The electrode had a very short response time (<10 s) and good reproducibility. There was no significant change in the slope of the concentration-potential curve during the usage period of 10 weeks. The developed electrode was utilized for the determination of lithium contents of human blood serum samples and pharmaceutical formulations in steady-state conditions. The obtained potentiometric results for both serum samples and pharmaceutical formulations are in good agreement with atomic emission spectrophotometry (AES) results at the 95% confidence level. (C) 2008 Elsevier B.V. All rights reserved.