Spectra and compartmentalization of digestive proteinases and their inhibitors (PI) were studied in the midgut of insects from 3 different orders: Nauphoeta cinerea (Olivier) (Blattodea:Blaberidae); Tenebrio molitor Linnaeus (Coleoptera: Tenebrionidae); and Ephestia kuehniella (Lepidoptera: Pyralidae).  N. cinerea and T. molitor possess a sharp gradient from acidic to alkaline pH between posterior (PM) and anterior (AM) midgut.  However, the total proteolytic activity in N. cinerea midgut is located mainly in the PM, and consists of an unusual SH-dependent proteinase, trypsin-, chymotrypsin- and subtilisin-like proteinases, all with highly alkaline pH-optima.  Proteolytic activity in the AM is low because of (1) acidic pH of contents and (2) presence of proteinase inhibitors which loose 40-80% of their activity in the alkaline medium of the PM.  In the midgut of T. molitor larvae, 64% of the total proteolytic activity is located in the AM. The major activity in the AM is due to cysteine proteinases with pH-optima 5.0-7.0, while in the PM, serine proteinases, including trypsin-, chymotrypsin- and subtilisin-like, are more active. The activity of cysteine proteinases is unstable in the alkaline environment of the PM.  Two types of PI are found in the AM of T. molitor, but they do not affect the activity of cysteine proteinases and do not retard digestive proteolysis in the AM. In E. kuehniella larvae, pH is alkaline throughout the whole midgut. The activity of digestive proteinases also is predominate in the AM (67%). Only serine proteinases are observed in this insect, with trypsin-like activity found in the AM and PM. No PIs are found in the midgut of these larvae.  Our data suggest a correlation between a sharp pH gradient and the presence of PI in the midgut of an insect.  Supported by RFBR (02-04-48808,-06347) and CRDF (RB2-2396-MO-02).