1. The saturable binding of exchange-labelled tritiated saxitoxin (STX) to the extensor digitorum longus (e.d.l.), diaphragm and soleus muscles of adult rats was studied. By measuring STX uptake to small pieces of muscle, the dissociation constant (KD) and binding capacity could be determined for individual muscles. 2. The affinity for STX is very similar in all three muscles, with a KD of 4.3 +/- 0.3, 5.1 +/- 0.5 and 4.9 +/- 0.5 nM (mean +/- S.E. of mean) at 4 degrees C for e.d.l., diaphragm and soleus respectively. The maximum binding capacity, which varies between different muscles, is 52.6 +/- 2.5, 40.3 +/- 4.9 and 23.8 +/- 1.1 f-mole.mg wet wt.-1 respectively. 3. The affinity of e.d.l. for tetrodotoxin (TTX), measured by inhibition of STX binding, is 12.1 +/- 1.4 nM at 4 degrees C. Raising the temperature to 37 degrees C increases the KD for STX to 6.8 +/- 0.8 nM and the KD for TTX to 47.5 +/- 4.5 nM. 4. STX binding is pH dependent; protons compete with STX for the binding site as if there were a titratable acidic group with a pK of 5.5. 5. The binding capacity of the diaphragm is not uniform along the length of the muscle fibres. Binding at the ends of the fibres is only 78% of that in the central region. 6. Denervation of e.d.l. for 7 days causes no change in the affinity for STX. There is a slight reduction in the binding capacity from 54 +/- 5 to 43 +/- 3 f-mole.mg wet wt.-1. There is no change in the diameter of the muscle fibres.