A randomised controlled clinical trial was conducted to investigate the ability of nicotinic acid to reduce intestinal secretion in patients with severe cholera. Of the 62 adults investigated, 29 received either 1 or 2 g of nicotinic acid given orally in divided doses and 33 served as controls. Patients who received the 2 g dose had less fluid loss than did their controls during the first (p less than 0.01) and second (p less than 0.05) 8 h post-treatment periods. During the third and fourth 8 h periods, the rates were lower in the treatment groups, but not significantly so. The drug-specific stool reduction was 31%-47% during the first 16 h. Patients receiving 1 g consistently had lower rates of purging than had their controls during each 8 h observation period, but the differences were not significant. The effect of the 2 g dose was significantly better than that with the 1 g dose. The peak inhibition occurred 8-16 h after start of therapy. The drug was well tolerated, the only side-effect being transient flushing of the body in 1 patient.