1. There is a time lag between the beginning of sweat secretion and fully effective production of fluid of minimum salt content. It is suggested that changes in permeability to water or electrolytes have to be effected and may account for the time lag.
2. The composition of thermal and pilocarpine sweat indicates that pharmacological stimulation does not wholely reproduce the physiological mechanism: pilocarpine sweat contains more sodium and more cyclic AMP, irrespective of the secretory rate.
3. The sweat obtained from patients with cystic fibrosis has been compared with that from normal children: the concentrations of cyclic AMP and cyclic GMP are the same. Defective sodium reabsorption in the patients is therefore unlikely to be due to inadequate or excessive synthesis of these cyclic nucleotides.