Transistors are often envisioned as alternative transducing devices to microelectrodes to communicate with the nervous system. Independently of the selected technology, the transistors should have reliable performance when exposed to physiological conditions (37A degrees C, 5% CO2). Here, we report on the reliable performance of parylene encapsulated indium gallium zinc oxide (IGZO) based thin-film transistors (TFTs) after prolonged exposure to phosphate buffer saline solution in an incubator. The encapsulated IGZO TFTs (W/L = 500 mu m/20 mu m) have an ON/OFF current ratio of 10(7) and field effect mobility of 8.05 +/- A 0.78 cm(2)/Vs. The transistors operate within 4 V; their threshold voltages and subthreshold slope are similar to 1.9 V and 200 mV/decade, respectively. After weeks immersed in saline solution and at 37A degrees C, we did not observe any significant deterioration in the transistors' performance. The long-term stability of IGZO transistors at physiological conditions is a promising result in the direction of metal oxide bioelectronics.