The effect of operational parameters such as hydraulic retention time (HRT), influent substrate concentration (So), pH, temperature and recycling on the acidogenic digestion of a pharmaceutical biomass was investigated. The total COD content of the pharmaceutical effluent used was 50 g/litre, only 13.6% occurred in soluble form and consisted mostly of protein (22 g/litre). Substrate degradation was quantified in terms of COD, protein and carbohydrate degradation. Variations in COD solubilization were mainly due to protein solubilization. The best results, in terms of COD solubilization and concentrations of produced VFA, were obtained at pH = 8.5, T = 35 degrees C and OLR = 5.4 gCOD/litre/day. Under these conditions 50.6% of the initial COD content was solubilized and 17.1 g/litre of volatile fatty acids (VFA) were produced. Recycling of biomass resulted in greater COD solubilization but in lower acidogenic activity. Moreover, the effluent of the acidogenic reactor showed poor settling properties.