['Growth of the small intestine in the infant rat is promoted by crypt fission and later by increased crypt cell proliferation. Notch signaling could promote crypt fission. Hes-1 is a Notch target gene.', 'We assessed the effect of Notch signaling on intestinal crypt fission and on growth of the intestine in the infant rat.', "Hes-1 expression was determined in the small intestine of litters of Hooded Wistar rats aged between 3 and 72\xa0days. Hes-1 RNA expression was measured by quantitative RT-PCR. Four groups of rats (n\xa0=\xa08 or 9) were injected daily, ip, either with vehicle or with the Notch inhibitor DAPT at doses of 3, 10, and 30\xa0mg/kg, from days 9 to 13 of life, and killed on day 14. A microdissection technique was used to measure crypt fission, mitotic count, and apoptotic count. Data were analyzed by ANOVA and by use of Dunnett's F test.", 'Hes-1 expression and crypt fission peaked on day 14. DAPT reduced Hes-1 immunostaining in proportion to dose. DAPT reduced villous area to 72\xa0% (p\xa0<\xa00.01), 53\xa0% (p\xa0<\xa00.001), and 38\xa0% (p\xa0<\xa00.001) of control values for 3, 10 and 30\xa0mg/kg doses, respectively, and reduced crypt fission to 53\xa0% (p\xa0<\xa00.001) and 38\xa0% (p\xa0<\xa00.001) of control values, respectively, for 10 and 30\xa0mg/kg doses. Crypt mitotic count was not affected by any DAPT dose. DAPT at 10 and 30\xa0mg/kg significantly increased apoptosis in crypts, by 6.5 and 4.8-fold, respectively.'] Does notch signaling promote intestinal crypt fission in the infant rat?