AbstractThe respiratory increase observed in potato‐tuber tissue after infection withGibberella saubinetiimight be related to the synthesis of proteins in the host. Therefore the alcohol‐soluble and ‐insoluble N‐fractions have been determined in non‐infected and infected tuber halves in samples cut at various distances from the surface. Samples were cut starting from the surface. Consequently the samples at 0‐0.5 mm from the surface contained mycelium of the invading parasite; the other samples were mycelium‐free.In the samples at 0‐0.5 mm from the surface of the infected halves, and to a lesser extent in the corresponding samples of the non‐infected halves the alcohol‐insoluble N increased while the alcohol‐soluble N decreased. The total N‐content was unchanged. Apart from a wound‐reaction which may have occurred during the incubation period due to the cutting of the tubers into two halves before inoculation of one half, it is most likely that the shift in the ratio insoluble N/soluble N of the samples at 0‐0.5 mm from the infected surface is the consequence of the presence of mycelium in the tissue disks.As no change in soluble N and insoluble N could be demonstrated in the samples cut at 0.5 mm or more from the surface, no evidence has been obtained for a parallel behaviour of the respiration rate and the synthesis of insoluble nitrogenous compounds in the host tissue after infection. The possibility that synthesis and breakdown of proteins occurred simultaneously without any change in the total amount