The effects of hypertrehalosaemic octapeptides (peptides from AKH family) on transcription and trehalosegenesis in the fat body of adult females of Tenebrio molitor L. were examined. A synthetic hypertrehalosaemic peptide from CC of Periplaneta americana periplanetin CC-2 (Pea-CAH-II) Glp-Leu-Thr-Phe-Thr-Pro-Asn-Trp-amide (1), its new synthetic analogues Glp-Leu-Thr-Phe-Thr-Pro-Asn-Phe-amide (2), Glp-Leu-Thr-Phe-Thr-Pro-Asn-D-Trp-amide (3), Glp-Leu-Thr-Phe-Thr-Pro-Asn-D-Phe-amide (4), and an octapeptide from T. molitor Glp-Leu-Asn-Phe-Ser-Pro-Asn-Trp-amide (Tem-HrTH) (5) were tested. Peptides 2, 3 and 5 suppress transcription in the fat body tissue measured as incorporation of [3H]-uridine into total RNA. The rate of RNA inhibition in the tissue depends on the peptide concentration. Periplanetin CC-2 (2) was inactive in this test. Endogenic T. molitor octapeptide (5) stimulates trehalose release from the fat body of females, whereas Pea-CAH-II analogues (2-4) do not increase trehalose release from this tissue.