We examined the concentration-dependent blocking effects of intracellular Mg²⁺ on L-type Ca²⁺ channels in guinea-pig ventricular myocytes, using the whole-cell variation of the patch clamp technique. Mg²⁺-dependent block of I_Ca was indicated by an increase in peak amplitude of I_Ca with reduction of [Mg²⁺] in the intracellular solution. The increase of I_Ca (due to relief of Mg²⁺ block) occurred in two temporal phases. The rapid phase (run-up) transiently appeared early (< 5 min) in dialysis of the low-Mg²⁺ solution; the slow phase began later in dialysis (usually beyond 10 min). Run-up was more apparent at low temperature (24°C) than at high temperature (32°C), and was not blocked by intracellular GTP. The late phase of increased I_Ca (induced by intracellular dialysis of low Mg²⁺) was suppressed by intracellular GTP (0.4mM), and was observed in myocytes of the guinea pig or frog at higher (32°C or 24°C, respectively) rather than lower temperatures (24°C or 17.5°C, respectively). At pMg = 6.0, raising the temperature from 24 to 32°C evoked the late phase of increased I_Ca with a Q₁₀of 14.6. Restoring the temperature to 24°C decreased the late phase of I_Ca (relative to its elevated level at 32C°) with a Q₁₀ of only 2.4. The marked difference in the Q₁₀ values indicated that the late phase of increased I_Ca (pMg = 5-6) is an irreversible phenomenon. Phosphorylation suppressed the [Mg²⁺]_i dependency of the late phase of increased I_Ca, which was consistent with previous results in frog ventricular myocytes. This effect of phosphorylation, together with the inhibitory action of GTP on Mg²⁺-dependent blocking of I_Ca, are common properties of mammalian and amphibian cardiomyocytes.