A Nova 1 potassium analyser has been evaluated for the first 3 months of its use in a cardiac surgical intensive care unit. The machine was out of action for 11 out of 104 days. 515 external calibrations were performed with 4.50 mmol/l potassium standards: mean 4.49 mmol/l, standard deviation 0.07. 692 comparisons were made with routine laboratory flame photometer measurements on the same sample: correlation coefficient r = 0.815. It is concluded that Nova 1, if kept in working order, can provide potassium estimations quickly and accurately.