1. The aim of this study was to characterize further the two main metabolic pathways of regulation of the Na+-Ca2+ exchanger in squid axons induced by its two naturally ocurring high-energy compounds: ATP and phosphoarginine (Pa). [Na+]o-dependent Ca2+ efflux (forward Na+o-Ca2+i exchange) and [Ca2+]o-dependent Ca2+ efflux (Ca2+o-Ca2+i exchange) were measured in internally dialysed squid axons at 16-17 C. 2. Measurements of changes in the apparent affinity of the Na+-Ca2+ exchanger for transporting (Na+o, Na+i, Ca2+o, Ca2+i) and regulatory (Ca2+i) ions induced by ATP and Pa show marked differences for the two substrates: (i) ATP strongly alters the affinity for Na+o and Na+i, while Pa does not, and (ii) in the absence of Na+i, ATP has no stimulatiory effect; on the other hand, Pa causes a dramatic increase in Na+o-Ca2+i exchange with little activation of Ca2+o-Ca2+i exchange. 3. The MgATP analogue chromium-ATP (CrATP) completely inhibits MgATP stimulation of the Na+-Ca2+ exchanger. Nevertheless, even with the effects of the nucleotide blocked, Pa exhibits its usual activation of the [Na+]o-dependent Ca2+ efflux. 4. None of the classical serine-threonine-tyrosine kinase inhibitors, nor the PP1 and PP2 phosphatase inhibitors, affects either the ATP or the Pa effect. However, intracellular microinjections of an exogenous phosphatase (alkaline phosphatase) completely reverses the stimulation of the Na+-Ca2+ exchange induced by ATP and Pa. 5. Prolonged intracellular dialysis with highly permeable porous capillaries (18 kDa molecular weight cut-off), which normally induces a complete run-down of the MgATP effect, does not alter the Pa stimulation of the exchanger, even after 6 h of continuous dialysis. 6. We conclude that the ATP and Pa modulation of Na+-Ca2+ exchange in an invertebrate nerve fibre are two genuinely different mechanisms, which affect the carrier properties in very different ways. An interesting similarity between ATP and Pa is that a phosphorylation-dephosphorylation process seems to be a common feature of these two regulation modes.