To investigate the effects of stressful stimuli on pineal gland activity, male Wistar albino rats (200-250 g, 2-4 per group) were submitted to 30 min of forced immobilization or to unilateral vibrissotomy 30 min before sacrifice. In situ hybridization was performed with a 35S-labelled 50-base oligonucleotide probe complementary to nucleotides 270-319 of rat c-fos on sections containing the pineal gland. Autoradiograms were quantified using a JAVA microdensitometer. Stressful stimuli induced a significant increase in the expression of c-fos mRNA in the pineal gland (restraint = 144.3 +/- 14.4 cpm/mm2; hemivibrissotomy = 206.7 +/- 29.5 cpm/mm2) as compared to no restraint animals (30.6 +/- 5.1 cpm/mm2), animals displaying tonic-clonic seizures after an ip (64 mg/kg) injection of pentylenetetrazole (34.0 +/- 4.7 cpm/mm2), or competition (70.6 +/- 11.4 cpm/mm2) and RNAase-treated (52.7 +/- 9.1 cpm/mm2) controls. These results raise the possibility that stressful stimuli may interfere with pineal gland function.