The effects of various durations (0.5, 2, 12, 48, or 120 h) of day- and night-time exposures to a 1.0 gauss (rms) 60-Hz magnetic field or sham field on mortality levels in the nocturnally-crepuscularly active land snail, Cepaea nemoralis, were examined. These snails were injected with morphine or saline vehicle and tested for reaction to an aversive thermal stimulus as part of another study. Mortality levels were monitored over a 2-week period following the initial exposure to the fields and were shown not to be differentially affected by the drug injection procedures. Mortality levels increased linearly as a function of increased length of exposure to the magnetic fields (P less than 0.001) but not when exposed to the sham fields. As well, night-time exposures resulted in greater mortality levels than day-time exposures (P less than 0.025). These results indicate that day-night rhythms are important in determining the magnitude of the magnetic field exposure effect. It is speculated that the magnetic fields may disrupt endogenous opioid- and calcium-modulated homeostatic mechanisms and augment stress effects, modifying a variety of systems including immunocompetence.