A technique based on release of calcium ions from in vitro preparations of avian brain tissues has been used by several investigators to demonstrate a biological effect of weak electric and magnetic fields. When the tissues have been exposed to ELF-modulated, VHF and UHF fields, enhanced release of calcium ions has resulted. Both inhibition and enhancement in release of calcium ions have been reported. Avian-brain tissues were exposed to 16-Hz sinusoidal electromagnetic fields at 14.1 V(sub rms)/m (in air) and 64 nTrms. During 20-min exposures, as tissue-sample temperature rose by 0.7 to 2.5 C to a temperature of 35, 36, or 37, but not of 38 or 39 C, an enhanced release of ions was observed. When the temperature was stable during exposure (i.e., constant within + or - 0.3 C) at a final value of 36 or 37, but not of 35 or 38 C, the quantity of ions released was reduced. And when descending by 0.7 to 1.5 C to any final temperature from 35 to 38 C, a null result occurred. These findings may reconcile the apparent disagreement in the direction of a field-induced response, and they may explain why experimental outcomes have been difficult to confirm in some laboratories. Of greater importance, the findings may also provide insight into the mechanism of the field-induced phenomenon.