In previous studies it has been suggested that the increase in the number of synaptic ribbons is parallel to the melatonin forming activity of the pineal gland. The presence of synaptic ribbons in the pinealocytes of rats subjected to experimental manipulations known as enhancers of pineal actions (blindness, blindness + olfactory bulbectomy, blindness + underfeeding and blindness + cold exposure) has been studied. We found a significantly lower number of synaptic ribbons in the pinealocytes of control rats as compared with rats subjected to enhanced pineal actions. Since the latter showed a pineal-dependent gonadal atrophy and low serum estradiol levels, a direct relationship between the number of synaptic ribbons and the functional status of the pineal gland may be established. Thus, our results suggest that the number of synaptic ribbons might be an indicator of pineal activity.