Some features of the chemical limnology of a hypertrophic, gravel-pit lake close to Madrid (Spain) have been studied at weekly intervals throughout a year. El Porcal Lake in an alkaline, slightly saline, water body without surface inflow or outflow. Anoxia is a common feature below the 2 m level from May to September. Dissolved oxygen oversaturation and pH above 9,00 occurred frequently in surface waters throughout the study period. Alkalinity totalled to 2,5-3,5 meq.l-1, bicarbonate being the main inorganic carbon fraction. Inorganic carbon is a key parameter in the chemistry of the lake, as indicated by statistical correlations. Nitrate and nitrite values were very high, probably as a result of high nitrification rates. Ammonia levels were lower than those of nitrite and nitrate. Soluble Reactive Phosphorus ranged over three orders of magnitude, sometimes occurring below detection limits. No plankton collapses have been observed but frequent oscillations in the chemical parameters have been the rule during the whole year. Groundwater seepage might be a very important source of nutrients. Most of the chemical features showed time lags of a week in autocorrelation functions, suggesting an unknown control mechanism for the temporal dynamics of the chemical limnology of El Porcal Lake.