Sand gobies, Pomatoschistus minutus, were collected monthly from September 2002 to August 2003 at a station situated 8 km upstream from the mouth of the Guadalquivir estuary (southwest Spain). Physical parameters of the water and selected biomarkers of organic pollution were recorded in the fish to discuss its potential as a sentinel species in estuaries. The biomarkers selected were the activities of catalase (CAT), 7-ethoxyresorufin-O-deethylase (EROD), and glutathione S-transferase (GST) in the liver and acetylcholinesterase (AChE) and lipid peroxidation (LP) in the head. The results showed an increase in total protein synthesis in late spring and early summer coinciding with the reproductive period as well as the release of fresh water from a dam situated 110 km upstream. During the same period, a significant depletion of hepatic GST and head AChE but higher LP levels in this tissue suggest exposure to pesticides such as those applied to crops established along the course of the river and reaching the estuary mostly when the freshwater discharges occur. Changes in CAT and EROD activities fluctuated randomly and were not noted as seasonally dependent. Biomarker fluctuations in sand goby are discussed as normal seasonal variations, but other variables-such as potential local pollution inputs-cannot be disregarded.