Evidence for a nervous connection between the brain and the pineal organ in the guinea pig.

Following the injection of horseradish peroxidase into the pineal organ of the guinea pig, approximately 30 nerve fibers were demonstrated in the pineal stalk due to the retrograde transport of the tracer enzyme in these elements. Finely branched extensions of these nerve fibers are directed toward the distal portion of the pineal organ. This projection of central nervous elements enters the pineal organ via the habenular or posterior commissures. Neuronal perikarya projecting into the pineal organ are found in the region of the paraventricular nucleus near the border of the third ventricle.