The relationship between the plasma level of the cardiac glycoside digoxin and performance on a range of cognitive tasks was investigated in a group of cardiac outpatients on maintenance therapy. The cognitive functions assessed were speed of decision-making and recognition memory of nonverbal and verbal material, competitive audio-visual attention performance, visuo-motor-choice reaction time and auditory verbal learning (AVL). Multiple regression analysis of the data revealed a strong relationship between plasma digoxin level at the time of testing and the number of trials to criterion in the AVL task (plasma digoxin level 0.7-1.8 nmol/l). Further data analysis indicated that this association was unlikely to be due to age, education or severity of cardiac disorder. The results strongly suggest that an increasing plasma level of digoxin may produce a progressive deterioration in auditory verbal learning and short-term memory.