The analysis of picomolar quantities of magnesium by electrothermal atomic absorption spectrophotometry (EAAS) was studied using a Perkin-Elmer-Zeeman 3030 spectrophotometer. The absorbance signal was not heavily dependent on the atomization temperature, but was greatly reduced when ashing temperatures in excess of 1200¡C were applied. The magnesium signal was significantly depressed in the presence of excess chloride in the sample matrix. However, use of NH4NO3 as a matrix modifier was sufficient to overcome this artefact. The analytical sensitivity was 0·15 absorbance units pmol-1 and the detection limit was 0·04 pmol. Using nanolitre constriction pipettes to dispense standards, the mean coefficient of variation was 5 %. Measurement of magnesium handling in the rat proximal convoluted tubule revealed a significant correlation between the tubular fluid-to-plasma ultrafiltrate (TF/UF) concentration ratio for magnesium and the tubular fluid-to-plasma (TF/P) concentration ratio for [3H]inulin (r2 = 0·56, n = 17). This indicated that magnesium is concentrated during its passage along the proximal tubule. In contrast, this was not the case for sodium (r2 = 0·11, n = 16). Mean (TF/UF)Mg (1·16 ± 0·07, n = 17) for random punctures was significantly greater than that for sodium ((TF/UF)Na = 1·02 ± 0·02, n = 16). Despite concentration of magnesium in the lumen, significant net reabsorption of magnesium was observed along the length of the tubule (fractional reabsorption, FRMg = 19·4 ± 3·0 %, n = 17). In conclusion, EAAS provides a highly sensitive, reproducible and technically simple method for measuring picomolar quantities of magnesium in renal tubular fluid.