The function of Merkel cells in mechanotransduction has remained controversial. Single unit recordings were made from Merkel cell receptors (sinus hair type I, St I) and another slowly adapting mechanoreceptor (sinus hair type II, St II) in isolated rat sinus hairs by applying controlled mechanical displacements to the hair shaft. Chloroquine (50-300 μM) caused a concentration dependent inhibition of Merkel cell receptor responses to mechanical stimulation. In contrast, both stimulated and spontaneous spike activity of St II receptors was increased by the same concentrations of chloroquine. Ultrastructural examination of chloroquine treated sinus hairs revealed swollen Merkel cells with multiple vacuoles and randomly distributed granules while other neural and surrounding structures showed no striking morphological changes. These results suggest that the Merkel cell plays a mechanotransducer role in Merkel cell receptors.