Pregnant Wistar rats were exposed to either 0.5 Hz Rotating Magnetic Fields (50 microT to 300 microT or 1.5 mT to 3.0 mT) or to sham field or control environments for 3 days before to 3 days after the day of birth. The adult male offspring that had been exposed perinatally to the RMF displayed significantly heavier testicle weights but fewer neurons within the medial preoptic nucleus (MPO) and caudal ventromedial hypothalamus relative to sham field controls (effect size about 30%). There was no statistically significant difference between the two ranges of RMF intensities for these measures. RMF-exposed rats relative to sham-field exposed rats did not differ with respect to spleen weights, body weights, or mounting (reproductive) activity. The heavier testicle weights replicated previous studies and suggest that permanent, selective alterations in neuronal density can also occur subsequent to perinatal exposure to relatively weak, extremely low frequency magnetic fields.