A new analgesic compound, neopam HCl, was studied for possible gastrointestinal blood loss liabilities by Cr-51 red cell tagging. It was compared with aspirin in usual therapeutic doses of 1.8 Gm per day. Dosage of nefopam HCl was 180 mg per day (two tablets t.i.d). Twenty healthy male volunteers had fecal blood loss determined after one-week crossover periods of drug investigation, each preceded by one-week no-drug control periods. Results show a significant (P is less than 0.01) increase in occult bleeding in the aspirin-treated subjects from a mean of 0.5 ml to 1.63 ml per 24 hours, the mean increase for the group being 1.13 ml per 24 hours. Nefopam HCl treated subjects had an insignificant change from a control mean of 0.55 ml per 24 hours to 0.60 ml per 24 hours; a third of this group actually had a decrease in measurable blood loss. Reported side effects were minimal in both drug groups.