The rear halves of the bodies of anaesthetized male C3H mice were exposed for 30 min to 2.45 GHz microwave radiation and the effects on the testes were compared to those produced by direct heating. Effects were observed which are consistent with the hypothesis that heat damage is the primary effect of microwave exposure. Damage measured six days after exposure ranged in severity from depletion of the spermatocytes to extensive necrosis of the germinal epithelium. Temperature-sensitive probes implanted in the testes revealed a threshold effect for depletion of the spermatocytes of approximately 39 degrees C and an LD50 6 (50 per cent cell death after 6 days) of about 41 degrees C after microwave exposure or direct heating. The corresponding effective threshold effect and LD50 6 expressed in terms of absorbed microwave power were 20 W kg-1 and 30 W kg-1. However, it is probable that a conscious animal is better able to regulate testicular temperature and hence adjust to higher dose-rates.