The performance of a laboratory-scale mesophilic acidogenic reactor was evaluated in this study, in terms of volatile fatty acid production and distribution, with respect to variations in hydraulic retention time (HRT). The continuous flow-completely mixed anaerobic reactor, coupled with a conventional gravity settling tank and a continuous recycling system, was operated in a hydraulic retention time ranging between 24 and 12 h, and up to an organic loading rate of about 9.3 kg COD m(-3) d(-1), without pH control. The acid production gradually increased proportionally to the organic loading rate, with decrease in hydraulic retention time. The highest degree of acidification and the rate of acid production were 56% and 3.1 g dm(-3) d(-1) at 12 h of HRT. Variations in hydraulic retention time affected volatile fatty acid production and distribution substantially, for the range investigated. Acetic, propionic, butyric and valeric acids were commonly produced during acidogenesis of dairy wastewater. (C) 2004 Society of Chemical Industry.