Effects of ammonium on the photosynthetic recovery of Nostoc flagelliforme Berk. et M. A. Curtis were assayed when being rehydrated in low-K+ or high-K+ medium. Its photosynthetic recovery was K+ limited after 3 years of dry storage. The potassium absorption of N. flagelliforme reached the maximum after 3 h rehydration in low-K+ medium but at 5 min in high-K+ medium. The K+ content of N. flagelliforme rehydrated in high-K+ medium was much higher than that in low-K+ medium. The maximal PSII quantum yield (Fv/Fm) value of N. flagelliforme decreased significantly when samples were rehydrated in low-K+ medium treated with 5 mM NH4Cl. However, the treatment of 20 mM NH4Cl had little effect on its Fv/Fm value in high-K+ medium. The relative Fv/Fm 24 h EC50 (concentration at which 50% inhibition occurred) value of NH4+ in high-K+ medium (64.35 mM) was much higher than that in low-K+ medium (22.17 mM). This finding indicated that high K+ could alleviate the inhibitory action of NH4+ upon the photosynthetic recovery of N. flagelliforme during rehydration. In the presence of 10 mM tetraethylammonium chloride (TEACl), the relative Fv/Fm 24 h EC50 value of NH4+ was increased to 46.34 and 70.78 mM, respectively, in low-K+ and high-K+ media. This observation suggested that NH4+ entered into N. flagelliforme cells via the K+ channel. Furthermore, NH4+ could decrease K+ absorption in high-K+ medium.