Scored for two oboes, two horns and strings with a bassoon continuo, Symphony No. 43 (called "Mercury" in the nineteenth century—no one knows exactly why) is one of Haydn's "chamber" symphonies—indicating not only the absence of drums and trumpets but also the style of writing. This type of symphony is usually of a lighter texture, with more lyrical themes than the so-called "grand" symphonies. Haydn would merge these characteristics to a great extent in his later symphonies, but this relatively early work maintains a distinct chamber quality. Dating from the composer's years in the service of the Esterházy family, it was most likely composed between the years of 1766 and 1772.

The thematic material at beginning of the symphony does not fall into the typical groups of two-, four- or eight-measure ideas. One of the longest Haydn ever wrote, the main theme is unabashedly lyrical and slow moving, underpinned by an unusual harmonic progression. Even the traditional tutti restatement and transition are not heavy; the orchestration is transparent with the horns working on the periphery. Once the secondary theme arrives, we find that it is really a varied restatement of the first theme, but on the dominant. The false recapitulation in the development section has something of the comic opera in it, while in the real recapitulation Haydn allows himself to dwell on the main theme even longer than in the exposition.

The second movement is luxuriant and performed mostly by the strings. Haydn spends an unusual amount of time spinning out the principle theme. Possibly because of this excess, the development section focuses not on the theme, but on a fragment consisting of three repeated sixteenth notes followed by two eighth notes that either rise or fall.

Harmonic contrast is paramount in the E flat major Minuet, which is clearly reminiscent of the Austrian Ländler. The graceful trio section begins in C minor, its first half moving to B flat (the dominant of the Minuet). This harmony does not go where we expect it to, for the second part of the trio begins again on C minor, moving eventually to E flat of the return of the Minuet. As in the slow movement, the winds are used sparingly.

Instead of a brisk, ebullient finale that might close one of the "grand" symphonies, Haydn writes for this work a Finale with a lengthy, legato theme. The coda is very unusual in that it does not end the movement with a "bang"; rather, it consists of lyrical passages that diminish both in dynamic and instrumentation, leading to a moment of silence. Haydn then calls for a final tutti passage before the finish.

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