The aim of this study was to investigate the effects of Lactobacillus fermentum Zhao (LF-Zhao) on activated carbon-induced constipation in ICR mice. ICR mice were administered lactic acid bacteria by gavage for 9 d. Body weight, diet intake, drinking amount, stool status, gastrointestinal transit distance and stool time, in addition to motilin (MTL), gastrin (Gas), endothelin (ET), somatostatin (SS), acetylcholinesterase (AChE), substance P (SP) and vasoactive intestinal peptide (VIP) levels in serum were monitored to evaluate the preventive effects of LF-Zhao on constipation. Bisacodyl, a laxative drug, was used as a positive control. Times to the first black stool for normal (untreated), control (no lactic acid bacteria treatment but activated carbon treated), bisacodyl-treated and L. delbrueckii subsp. bulgaricus (LB), LF-Zhao (L) (low concentration of 1×10(8) CFU/mL)- and LF-Zhao (H) (high concentration of 1×10(9) CFU/mL)-treated mice induced by activated carbon were 90, 218, 117, 180, 169 and 156 min, respectively. Following the consumption of LB, LF-Zhao (L) and LF-Zhao (H) or the oral administration of bisacodyl, the gastrointestinal transit distances were reduced by 55.2%, 61.3%, 70.6% and 94.6%, respectively. The serum levels of MTL, Gas, ET, AChE, SP and VIP were significantly increased and the serum levels of SS were reduced in the mice treated with LF-Zhao compared with those in the control mice (p<0.05). These results demonstrated that lactic acid bacteria demonstrate preventive effects on mouse constipation and that LF-Zhao alleviated constipation symptoms better than LB.