The release of calcium ions from in vitro preparations of chicken brain-tissue has been used by several investigators to demonstrate the interaction of electric and magnetic (EM) fields with biological systems. For exposures with radiofrequency radiation amplitude modulated at extremely low frequencies (ELF), the observations have all been of field-induced enhancement of the calcium release. In contrast, when only ELF EM fields are employed, the release of calcium ions has been reported by one research group to be inhibited and by another group, enhanced. The results in the report demonstrate that either observation, or a null result, is possible under exposure to 16-Hz sinusoidal EM fields, at 14.1 Vrms/m (in air), and 64 nTesla-rms, depending upon the thermal condition of the samples prior to and during exposure. When the sample temperature is ascending during the 20 minute exposure enhanced release is observed, when stable during exposure (i.e., within + or - 0.3C of the final temperature) the release is reduced, and when descending a null result occurs. This finding may reconcile the disagreement in the direction of the field-induced response and explain the reason the experimental result is so difficult to replicate.