This ETS defines the GSM Application Programming Interface (GSM-API) in two parts. The first part describes@ how compatibility to existing application interface ETS 300 838 [14] is covered for the GSM network. So existing PCI applications are able to be used in a GSM environment. For these applications the necessary mapping and local knowledge between application interface and network is described. The only modifications needed in ETS 300 838 [14] to fulfil this requirements are covered by changes of the parameter description. These changes of parameters are defined in clauses 4 and 5 of this ETS. Clause 4 is meant to replace subclause 5.7 message parameters of ETS 300 838 [14]. which defines the parameters of the profile A of the PCI. NOTE 1: Clause 4is for further study. Clause 5replace subclause 6.8 parameter description of ETS 300 325@ which defines the parameters of the profile B of the PCI (bit compatible to COMMON-ISDN-API@ CAPI 2.0). The second part defines GSM specific features. New GSM-API applications need extensions to ETS 300325 [14] which are defined in section 6 and 7of this ETS. These sections are meant as an addition to ETS 300325 [14]. They do not replace any clause of ETS 300325 [14]. Clause 6 defines the extensions according to the profile A of ETS 300325 [14]. NOTE 2: Clause 6 is for further study. Clause 7 defines the extensions according to the profile B of ETS 300325 [14] (bit compatible to COMMON ISDN API). The messages and the operating system dependent part of ETS 300325 [14] will not be changed for GSM-API.