A reverse transcriptase PCR (RT-PCR) targeting a 407 bp fragment of the nucleocapsid gene of bovine coronavirus (BCV) was developed for detection of BCV RNA in feces of experimentally inoculated cattle. The sensitivity and specificity of the RT-PCR were confirmed using tissue culture-adapted BCV strains and feces of 2 calves inoculated with BCV. Ten nonpregnant, BCV seropositive, adult dairy cows were inoculated with winter dysentery (WD) (n = 8) or calf diarrhea (CD) (n = 2) strains of BCV intranasally and orally (n = 2) or through a surgically-placed duodenal catheter (n = 8) with and without dexamethasone treatment or feeding ice water. The 6 cows inoculated with BCV intranasally and through a duodenal catheter (2 of 2 cows given CD BCV and 4 of 6 cows given WD BCV) developed mild diarrhea, and BCV was detected in diarrheal feces by RT-PCR, ELISA or immune electron microscopy. These results suggest that CD and WD strains of BCV can cause diarrhea in adult cows in conjunction with host or environmental factors and that RT-PCR might be useful to diagnose BCV infections in calves and adult cows.