I opened my mouth and panted. [1] By these words the Psalmist would have us to understand that he was inflamed with such love to and longing for the divine law, that he was unceasingly sighing after it. In comparing himself to such as are hungry, or to such as burn with parching thirst, he has used a very appropriate metaphor. As such persons indicate the vehemence of their desire by opening the mouth, and by distressful panting, as if they would suck up the whole air, even so the Prophet affirms that he himself was oppressed with continual uneasiness. The opening of the mouth, then, and the drawing of breath, are set in opposition to a cold assent to the word of God. Here the Holy Spirit teaches with what earnestness of soul the knowledge of divine truth is to be sought. Whence it follows, that such as make little or no proficiency in God's law, are punished by their own indolence or carelessness. When David affirms that he panted continually, he points out not only his ardor but also his constancy.

Footnotes

1 - The allusion, according to some, is to an exhausted or thirsty traveler in hot countries, who gasps and pants for the cooling breeze, or the refreshing fountain. According to others this is a metaphor, taken from an exhausted animal in the chase, which runs open-mouthed, to take in the cooling air, the heart beating high, and the muscular force being nearly expended through fatigue. In either view the language is extremely expressive, showing how intensely the Psalmist longed for the refreshment and delight which an acquaintance with the word of God affords. And if the "opening of God's words," mentioned in the preceding verse, means the expounding of them, David here points out his eager desire to hear God's word expounded.

I opened my mouth and panted - All this is the language of deep emotion. We breathe hard under the influence of such emotion; we open the mouth wide, and pant, as the ordinary passage for the air through the nostrils is not sufficient to meet the needs of the lungs in their increased action. The idea is, that his heart was full; that he had such an intense desire as to produce deep and rapid breathing; that he was like one who was exhausted, and who "panted" for breath. Compare the notes at Psalm 42:1.
For I longed for thy commandments - The word here rendered "longed" occurs nowhere else. It means to desire earnestly. See the notes at Psalm 119:20.

I opened my mouth, and panted - A metaphor taken from an animal exhausted in the chase. He runs, open-mouthed, to take in the cooling air; the heart beating high, and the muscular force nearly expended through fatigue. The psalmist sought for salvation, as he would run from a ferocious beast for his life. Nothing can show his earnestness in a stronger point of view.

I opened my mouth, and (c) panted: for I longed for thy commandments.
(c) My zeal toward your word was so great.

I opened my mouth, and panted,.... As a person out of breath does, through walking or running; he stops and pants, and opens his mouth, to draw in air to his relief: or as hungry and thirsty persons pant for food and drink, and open their mouths to receive it, before it can well be brought to them. So the psalmist panted after God, and communion with him; desired the sincere milk of the word; longed for the breasts of ordinances, and even fainted for the courts of the Lord, Psalm 42:1;
for I longed for thy commandments; for an opportunity of waiting upon God in the way of his duty; to hear his word, and attend his worship.

An ardent desire (compare Psalm 56:1-2) for spiritual enlightening, establishment in a right course, deliverance from the wicked, and evidence of God's favor is expressed
I opened my mouth, and panted--as a traveller in a hot desert pants for the cooling breeze (Psalm 63:1; Psalm 84:2).

Panted - I thirst after thy precepts, and pursue them eagerly.