AIM: To investigate the in vivo effect of beta-casomorphin-7on the regulation of gastric somatostatin and gastrin messenger RNA in rat gastric mucosa.METHODS: Somatostatin and gastrin mRNA were quantified by RT-PCR and in situ hybridization (ISH)in 24 rats. The rats were divided into three treatment groups: basal diet + physiological saline (n = 8), basal diet + beta-casomorphin-7 (7.5 × 10-7 mol) (n = 8),and basal diet + poly-Gly-7 (containing equal mol of N with 7.5 x 10-7 mol beta-casomorphin-7) (n = 8).After oral administration for 30 days, rats were killed by exsanguinations.RESULTS: After intra-gastric administration of betacasomorphin-7 for 30 d, gastrin mRNA increased by 52.8% (P ＜ 0.05, n = 8), and somatostatin mRNA levels decreased by 30.7% compared with the controls (P ＜0.01, n = 8). No significant differences in the expression of the two genes were observed in the poly-Gly-treated group, although gastrin mRNA expression was elevated by 35.6% as against the control group (P = 0.15, n =8). The long-term oral administration of a casomorphin solution significantly decreased the even gray of D-cells,but did not lower the number of D-cells both in the antrum and fundus. Interestingly, the number of G-cells increased in the antrum and fundus, but its average density was augmented only in the antrum.CONCLUSION: Beta-casomorphin-7 is capable of modulating gene expression of the regulatory peptides from G and D cells. Data from in situ hybridization studies indicate that beta-casomorphin-7 affects gastrin gene expression indirectly by means of the paracrine action of somatostatin, and depends on its intrinsic molecular function.