Laboratory experiments were made to ascertain whether an observed mortality of herring eggs under natural conditions could have been due to respiratory difficulties caused by low CO2 or high O2 content of the water. In water saturated with O2, developing embryos tolerated a wider range of pH and CO2 values than is prevalent in the sea. Hence the spawning ground mortality is presumed not to be due to such factors. Low pH retarded development of embryos and prevented successful hatching.