1. The effects of ATP in the presence of theophylline and imidazole were investigated on the twitch tension of a partially magnesium blocked rat phrenic nerve-diaphragm preparation. Both theophylline and imidazole facilitate the neuromuscular transmission and prevent the effects of ATP. 2. The effects of adenosine in solutions with low calcium concentrations in the frog-sartorius and in solutions with very low calcium concentrations in the rat-diaphragm were studied on the miniature end-plate potentials. Adenosine caused a similar reduction of the frequency of the miniature end-plate potentials in both low and very low calcium concentrations. 3. The results are discussed in relation to the cyclic AMP and calcium hypothesis.