We have studied the ultrastructure of the pinealocyte after 24 h of immobilization. Thirty male albino rats (230 ± 10 g) were used; the animals were divided into two groups of 15 each. One group was immobilized for 24 h (10.00 h a.m. to 10.00 h a.m.), whereas the other group was used as control. All animals were sacrificed between 10.00 and 14.00 h of the following day. Ultrastructural observations showed an euchromatinic nucleus with numerous giant ‘lipid droplets’ as well as abundant long ‘synaptic ribbons’ that were significantly more abundant than in the controls (p < 0.0005). These findings all suggest an increase in pineal activity following 24-hour immobi