1. The permeability of perfused intimal sacs to fourteen non-ionic, hydrophilic molecules of graded molecular size was estimated by radioisotope flux.

2. The rectal cuticle acts as a molecular sieve severely restricting the rate of penetration of molecules with increasing hydrated size.

3. The penetration of test molecules was as predicted by the Renkin equation for a uniform population of water-filled pores having radii of 6.5 Å.

4. The properties of cuticles from the rectum and the integument are compared and the role of the rectal intima in the excretory process in the desert locust is discussed.

This work was supported by Grants from the National Research Council of Canada.