We define an evolving in time Bayesian neural network called a Hidden Markov neural network. The weights of the feed-forward neural network are modelled with the hidden states of a Hidden Markov model, the whose observed process is given by the available data. A filtering algorithm is used to learn a variational approximation to the evolving in time posterior over the weights. Training is pursued through a sequential version of Bayes by Backprop, which is enriched with a stronger regularization technique called variational DropConnect. The experiments are focused on streaming data and time series. On the one hand, we train on MNIST when only a portion of the dataset is available at a time. On the other hand, we perform frames prediction on a waving flag video.