The effects of hydraulic retention time and alkali treatment on methane production rate from the semicontinuous anaerobic digestion of 2% sunflower-head/ water mixtures were investigated. The experiments were carried out in laboratory-scale fermenters, fed with I liter of untreated, 2 g NaOH/l00 g total solids (TS), and 5 g NaOH/100 g TS alkali-treated sunflower-head/water mixtures, respectively, and maintained at 55 °C. Digestion experiments were performed for hydraulic retention times of 8, 10, and 15 days. The amount and composition of produced gas were measured until steady state was attained in each run. The steady-state methane production rates were found to decrease with hydraulic retention time and increase with alkali dosage used for pretreatment.