Measurement of sodium in the nanoliter samples obtained from in vitro studies of isolated, perfused nephron segments has traditionally been associated with time-consuming sample preparation and/or expensive instrumentation. A new instrument is described which can measure picomole quantities of sodium without the need for special processing. The instrument is based on the continuous flow devices developed by Vurek and a sodium-sensitive glass detector. The response to varying concentrations of NaCl was logarithmic as predicted from the Nernst equation. The response to 50 mM NaCl was 22.1 +/- 0.2 mV; 100 mM, 29.9 +/- 0.2 mV; 150 mM, 35.2 +/- 0.2 mV; and 200 mM, 39.3 +/- 0.6 mV (mean +/- SD). The slope of the standard curve was 27.8 mV/log unit change in sodium concentration. Potassium, calcium and ammonium, possible interfering ions, had no effect on the response to 100 mM NaCl. When the response to 100 mM NaCl was measured in buffered solutions of varying pH, it was 29.1 +/- 0.2 mV at pH 6.21, 28.9 +/- 0.4 mV at pH 7.04 and 28.7 +/- 0.4 mV at pH 8.54, indicating that pH did not alter the response to 100 mM NaCl. By treating the detector with NaOH and HCl, the response to 50, 100 and 150 mM improved to the point that the slope of the standard curve was 51.2 mV/log unit change in sodium concentration. The calculated resolution after treatment was 1.0 mM in the 50 to 100 mM range and 1.3 mM in the 100 to 150 mM range using a 10.9 nl pipet.(ABSTRACT TRUNCATED AT 250 WORDS)