1. ACh release from motor nerve endings in diaphragms of rats treated chronically with alpha-bungarotoxin (alpha-BuTX) is upregulated at the level of the individual endplate. Involvement of protein kinases in this mechanism of synaptic adaptation was investigated. 2. Miniature endplate potentials (MEPPs) and endplate potentials (EPPs) were recorded after mu-conotoxin treatment, which prevents muscle action potentials. The quantal content at endplates was calculated 'directly', i.e. by dividing the EPP amplitude by the MEPP amplitude. 3. Incubation of muscles from control and alpha-BuTX-treated rats with H-7, a protein kinase C (PKC) inhibitor, reduced MEPP amplitudes but had no clear effect on quantal contents. Polymyxin B, another PKC inhibitor, had a similar effect on muscles from alpha-BuTX-treated rats. 4. Incubation of muscles from alpha-BuTX-treated rats with K252a, a broad-spectrum protein kinase inhibitor of, amongst others, PKC, Ca(2+)-calmodulin-dependent protein kinase II (CaMKII) and neurotrophin receptor tyrosine kinases, resulted in a 30% decrease of the quantal content. However, K252a did not change the quantal content of controls. Incubations with the closely related compound K252b, which has an exclusively extracellular action, had a similar effect. 5. KN62, a specific inhibitor of CaMKII, decreased the mean quantal content of muscles from alpha-BuTX-treated rats by 18%. 6. Tyrphostin 51, a selective tyrosine kinase inhibitor, had no effect on quantal contents of muscles from alpha-BuTX-treated and control rats. However, it increased the frequency and amplitude of MEPPs in muscles from alpha-BuTX-treated rats, leaving those of controls unchanged. 7. The extent of reduction of quantal content, caused by K252a, K252b and KN62, varied between endplates of individual muscles from alpha-BuTX-treated rats; quantal contents at endplates with small MEPPs were more sensitive than those at endplates with large MEPPs. 8. It is concluded that PKC does not play a role in the mechanism of upregulation of ACh release at endplates of alpha-BuTX-treated rats. Instead, CaMKII and some tyrosine kinases in the presynaptic membrane, as well as in the cytoplasm, might be involved.