The transport of sodium and chloride across human jejunal and ileal mucosa was studied using an in vitro technique. Specimens of mucosa removed at operation were stripped of muscle coats, mounted in specially designed Perspex flux chambers and bathed in warmed oxygenated and stirred buffer solutions. Evidence was obtained for the active transport of sodium in both jejunum and ileum and of chloride in the ileum. Sodium absorption was enhanced by glucose in both regions of the gut but net chloride transport was unaffected. Glucose had a greater effect on sodium transport in the ileum than the jejunum. The electrical potential difference and resistance was greater and undirectional ion fluxes smaller in jejunal than ileal mucosa. Many of these results with human intestine are similar to results reported with in vitro animal intestine. Apparent discrepancies between the behavior in vivo of human intestine and in vitro of animal intestine are thus likely to be due predominantly to technical rather species differences.