Although extremely low frequency (ELF) magnetic fields (<300 Hz) appear to exert a variety of biological effects, the magnetic field sensing/transduction mechanism(s) remains to be established. Here, using the inhibitory effects of magnetic fields on endogenous opioid peptide-mediated "analgaesic" response of the land snail, Cepaea nemoralis, we addressed the mechanism(s) of action of ELF magnetic fields. Indirect mechanisms involving both induced electric fields and direct magnetic field detection mechanisms (e.g., magnetite, parametric resonance) were evaluated. Snails were exposed to a static magnetic field (B(DC) = 78 +/- 1 mu T) and to a 60 Hz magnetic field (B(AC) = 299 +/- 1 mu T peak) with the angle between the static and 60 Hz magnetic fields varied in eight steps between 0 degrees and 90 degrees. At 0 degrees and 90 degrees, the magnetic field reduced opioid-induced analgaesia by approximately 20 percent, and this inhibition was increased to a maximum of 50 percent when the angle was between 50 degrees and 70 degrees. Because B(AC) was fixed in amplitude, direction, and frequency, any induced electric currents would be constant independent of the B(AC)/B(DC) angle. Also, an energy transduction mechanism involving magnetite should show greatest sensitivity at 90 degrees. Therefore, the energy transduction mechanism probably does not involve induced electric currents or magnetite. Rather, our results suggest a direct magnetic field detection mechanism consistent with the parametric resonance model proposed by Lednev.