We present a model for Rx ISI effects using a new and completely general and rigorous dynamic Rx model for-mulation aiming at 10–40 Gbit/s WDM system design applications. The signal shaping part of an electrical fil-ter is modelled in the optical domain using the filter impulse response to modify the signal vector and the covariance matrix. In this way inter-symbol-interference (ISI) effects from any type of electrical filter – specified by the filter impulse response – can be described. The formulation leads to a Hermitian form of the covariance matrix, thus allowing a straightforward analytical formu-lation of the moment generating function (MGF) which in turn specifies the bit-error-ratio (BER). Numerical ex-amples for a 10 Gbit/s pre-amplified Rx shows that the ISI leads to (up to) several dBʼs degradation in sensitivity. Thus it is imperative to account for the Rx ISI effect in rigorous and accurate form in any general WDM trans-mission model.