The lacrimal nerve, a branch of the trigeminal, is mainly a sensory nerve but it also conveys parasympathetic and probably sympathetic fibres to the lacrimal gland (Mutch, 1944). The role of these autonomic fibres in controlling lacrimal secretion is not clear. Stimulation of the lacrimal nerve in dogs was found by Tepliachine (1894) to cause an increase in lacrimal secretion. Botelho (1964) has shown that in cats and rabbits the secretory fibres are derived from the parasympathetic nervous system. In contrast, stimulation of the cervical sympathetic nerves produced variable effects on lacrimal secretion in man and animals (Wolferz, 1871;Reich, 1873;Schirmer, 1909;Duke-Elder, 1932; Whitwell, 1961; Botelho, 1964).The experiments described in this paper were undertaken to investigate the effects produced on lacrimal secretion in the cat by stimulation of the lacrimal nerve at different frequencies and to determine whether the secretory fibres were cholinergic. METHODSCats (2 to 4 kg of body weight) were anaesthetized with pentobarbitone sodium (35 mg/kg, intraperitoneally).Exposure of the lacrimal nerve presented a difficult problem; the most suitable approach consisted in removing the zygomatic arch after detaching the muscles from it. The nerve could then be located on the lateral aspect of the globe (Fig. 1). A ligature was tied round the lacrimal nerve as far centrally as possible and the nerve was cut proximal to the ligature. Further dissection in preliminary experiments had shown that a small branch of the lacrimal nerve supplied the lacrimal gland and that a larger branch appeared to innervate the superficial structures over the orbit. Since, however, section of the larger branch did not appear to modify the response of the lacrimal gland to nerve stimulation, it was left intact. Thus the lacrimal nerve could be exposed and stimulated at a site where it was readily accessible without dissection or exposure of the lacrimal gland.Rectangular wave stimuli of supramaximal intensity and 1.5 msec duration were applied to the peripheral end of the cut lacrimal nerve at frequencies ranging from 2 to 30 shocks/sec through bipolar platinum electrodes covered with liquid paraffin (B.P.). The nerve was stimulated for 5 min at a selected frequency and then allowed to rest for 10 min. This sequence was repeated for each of the frequencies used.The effects of nerve stimulation on lacrimal secretion were compared by a method similar to that described by Schirmer (1909). This consisted of placing into the superior fornix a suitably bent strip of Whatman No. 1 filter paper, 5 mm wide and 5 cm in length. To reduce loss by evaporation, the paper was enclosed in a plastic sheath. The strip was left in situ from the beginning of stimulation until immediately before the next stimulation period (a total of 15 min).