The effects on the composition of rat submandibular saliva, of superimposing low frequency sympathetic nerve stimulation on parasympathetic nerve stimulation have been investigated. During continuous parasympathetic stimulation, additional sympathetic stimulation resulted in a frequency-dependent rise of both protein and K+ concentrations. The threshold of frequency of stimulation for the protein response was 0.1 Hz and for the K+ response 0.4 Hz. The protein secretion is presumed to be acinar in origin whilst the K+ is believed to be secreted mainly by ducts other than the main excretory duct.