The effect of hydraulic retention time (HRT) on biohydrogen production from glucose in an internal circulation (IC) reactor with granular sludge as inoculants was studied. A maximum hydrogen-production rate (HPR) of 10.78 +/- 0.84 L(-1)d(-1) at HRT 2 h and a maximum hydrogen yield (HY) of 1.4 +/- 0.21 mol mol(-1) glucose at HRT 3 h were achieved. With no external pH adjustment, the effluent pH maintained at 5.16-3.71 under the carbonate buffer in the substrate. Regardless of the different HRTs, ethanol was the predominant metabolite and accounted for 36.59-48.98% of the total soluble microbial products (SMPs), which lead to a total IC reactor energy conversion rate of 11.42 +/- 4.15 kJ h(-1) L-1 at HRT 2 h based on the hydrogen and ethanol production. This means that the IC reactor was a feasible instrument in improving energy productivity.