The European Central Bank bought €348m worth of corporate bonds in a single day last week.
The figure well exceeded market expectations and signalled the ECB's commitment to the latest measure designed to revive inflation.
The purchase volumes, which would equal more than €7 billion per month at the first day's pace, are unlikely to be sustained.
But they do indicate that the ECB is keen to confound sceptics who said it would be hard to buy more than just a few billion euros each month.
Unveiled in March as part of its €1.74 trillion euro scheme to revive growth and inflation, the ECB started buying investment grade, non-bank corporate bonds last week to make borrowing cheaper and induce companies to spend, lifting a still sluggish euro zone economy.
Corporate borrowing costs have declined sharply since the start of the year, particularly on the edges of the 19-member euro zone, with the start of ECB buys giving the markets another push.
The ECB was seen in the market buying a wide range of corporate debt, including from Italian insurer Generali, Spain's Telefonica and French utility Engie.
ECB sources earlier said that buying corporate bond buys could struggle to pick up speed during the summer months as liquidity tends to fall sharply during the peak holiday periods.
But the purchases under the ECB's €80 billion per month asset-buying programme could then rise to around €2-3 billion per month and possibly over €5 billion if the ECB succeeds in getting companies to start issuing new debt, analysts say.
The key hurdle is that the market for investment-grade euro-denominated corporate bonds is worth €500-600 billion and tends to be dominated by big French and Dutch companies who already enjoy easy access to credit and may not need ECB cash.
For purchase volumes to ramp up, the ECB will need to get new issuers from the periphery such as Italy or Spain to start borrowing, issuing new debt, giving the ECB a chance to buy bigger chunks in the primary market.