Amphibian neuromuscular junctions (NMJs) are composed of hundreds of neurotransmitter release sites which exhibit non-uniform transmitter release probabilities and demonstrated seasonal modulation. We examined whether recruitment of release sites is variable when the extracellular calcium concentration ([Ca²⁺]_o) is increased in the wet and dry seasons. The amount of transmitter released from the entire nerve terminal increases by approximately the 4^th-power as [Ca²⁺]_o is increased. Toad (Bufo marinus) NMJs were visualised using DiOC₂(5)-fluorescence and focal loose patch extracellular recordings were used to record the end-plate currents (EPCs) from small groups of release sites. Quantal content (_e), average probability of quantal release (p_e) and the number of active release sites (n_e) were determined for different [Ca²⁺]_o. Our results indicated that the recruitment of quantal release sites with increasing [Ca²⁺]_o differs spatially (between different groups of release sites) and also temporally (in different seasons). These differences were reflected by the non-uniform alterations in p_e and n_e. Most release site groups demonstrated an increase in both p_e and n_e when [Ca²⁺]_o increased. In approximately 30% of release site groups examined, p_e decreased while n_e increased only during the active period (wet season). Although the dry season induced parallel right shift in the quantal release versus extracellular calcium concentration, when compared to the wet season, the dependence of quantal content on [Ca²⁺]_o was not changed. These results demonstrate the flexibility, reserve and adaptive capacity of neuromuscular junctions in maintaining appropriate levels of neurotransmission.