The neuromuscular effects of azathioprine were examined in thein-vivocat soleus muscle preparation. In concentrations ranging from 10 to 1,000 μg/kg. administered intra-arterially, the agent caused motor axons to fire repetitively and produced a dose-related increase in the force of contraction. The drug reversed neuromuscular blockade produced byd-tubocurarine and potentiated the neuromuscular blockade produced by succinylcholine. The effects of theophylline, a phosphodiesterase inhibitor, on neuromuscular transmission were identical to those produced by azathioprine. Using anin-ritroassay preparation, azathioprine was found to produce 50 per cent inhibition (IC30) of phosphodiesterase at a concentration of 2 × 10−3M. In the same preparation, theophylline had an IC30of 1 × 10−4M. Neither agent in concentrations to 10−2M affected cholinesterase activity measuredin ritro.It is concluded that the effects of azathioprine on neuromuscular transmission are due to inhibition of phosphodiesterase in the motor nerve terminal.