Total gaseous mercury in headspace air was measured for enclosed concretes dry curing at 40 degrees C for intervals of 2, 28, and 56 days. Release of mercury was confirmed for ordinary Portland cement concrete (OPC) and three concretes in which class F fly ash substituted for a fraction of the cement: (a) 33% fly ash (FA33), (b) 55% fly ash (FA55), and (c) 33% fly ash plus 0.5% mercury-loaded powdered activated carbon (HgPAC). Mean rates of mercury release (0.10-0.43 ng/day per kg of concrete) over the standard first 28 days of curing followed the order OPC < FA33 approximately FA55 < HgPAC. The mercury flux from exposed surfaces of these concretes ranged from 1.9 +/- 0.5 to 8.1 +/-2.0 ng/m(2)/h, values similar to the average flux for multiple natural substrates in Nevada, 4.2 +/- 1.4 ng/m(2)/h, recently published by others. Air sampling extending for 28 days beyond the initial 28-day maturation for OPC, FA55, and HgPAC suggested that the average Hg release rate by OPC is constant over 56 days and that mercury release rates for FA55 and HgPAC may ultimately diminish to levels exhibited by OPC concrete. The release of mercury from all samples was less than 0.1% of total mercury content over the initial curing period, implying that nearly all of the mercury was retained in the concrete.