Summary• Cord-forming woodland basidiomycete fungi form extensive, interconnected mycelial networks that scavenge nitrogen (N) efficiently. We have developed techniques to study N dynamics in such complex mycelial systems in vivo .• Uptake and distribution of the nonmetabolised, 14 C-labelled amino-acid analogue, α -aminoisobutyrate ( 14 C-AIB) was continuously imaged in Phanerochaete velutina growing across scintillation screens using an enhanced photon-counting camera.• Oscillations in the 14 C-AIB signal were observed for both the assimilatory hyphae in the inoculum and the foraging hyphae, but with complementary profiles. Pulses were asymmetric, with an abrupt switch between each exponential decay phase and the next rising phase. The period of the oscillations was 16 h at 21 ° C, but showed a strong temperature dependence with a temperature coefficient of 2.1. Oscillations occurred in the absence of obvious pulses in growth.• Some, but not all, of the features of the oscillations were simulated using a model of amino acid accumulation and transport that included both vacuolar uptake, and release once an intravacuolar concentration threshold was exceeded. The combination of imaging and modelling provides a useful framework to understand N fluxes in vivo .