Harmaline, a hallucinogenic alkaloid, inhibits sugar and amino acid transport by the guinea-pig intestinal mucosa in vitro during short and long incubations. It affects sodium-dependent transport systems in other tissues such as the dog colonic mucosa and renal cortex slices, but does not influence sodium-independent transport. It inhibits sodium entry into intestinal rings, both in the presence and absence of non-electrolytes. It is proposed that harmaline interacts with the sodium-binding site of the transport carriers in the membranes of intestinal and renal cells.