Rambam - Sefer HaMitzvos: Negative Mitzvot 144, 108: Negative Mitzvot 144, 108

Negative Mitzvah 144: It is forbidden for a Priest to eat the first-born sacrifice outside of Jerusalem
Deuteronomy 12:17 "You may not eat within your gates...the first- born of your herds or flocks"

The first-born animals must be redeemed or offered as sacrifices in the Beit Hamikdash (See positive Mitzvah 79).

The priest is entitled to portions of the meat that are not burnt.

A non-priest is not given a portion of this sacrifice.

This Negative Mitzvah applies only to a "perfect" first born animal, one that has no blemishes and is fit to be sacrificed.

This Negative Mitzvah contains two distinct laws.

The first is that a priest may not eat his portion of this first-born sacrifice outside of Jerusalem. The second is that a non-priest is forbidden to eat any of this meat at all.

Negative Mitzvah 108: We are forbidden to redeem first-born animals
Numbers 18:17 "But the first-born of an ox, or the first-born of a sheep, or the first-born of a goat you shall not redeem; they are holy"

At the time of the Exodus from Egypt, HaShem declared that all first-born male children and animals will be sacred to Him.

First-born male children may be "redeemed" in a special procedure called "Pidyon HaBen" (see Positive Mitzvot 79-80).

However, first-born oxen, sheep or goats are considered holy and must be offered as sacrifices. We are forbidden to redeem these first-born animals.

In truth, the world, standing on its own, is a place of exile and captivity. Even when a man stands upright on the tallest mountain and perceives all that there is to perceive, and comprehends all that can be comprehended, and achieves a realization of the ultimate oneness and void that is behind all this -- But in the end, he is still stuck on the ground where his feet have brought him, his eyes have not seen beyond his own eyeballs, his mind has only comprehended that which he has seen and reached that which it can reach -- he has remained within his own self.
And the proof -- he has remained with a G-d Who is above and an earth which is below, and the two cannot meet.
His only liberation, and the only liberation of the entire world, is when the One Above reaches down and tells us, "Do this. With this deed you are betrothed to Me." And then there is no above and below. Then there is only One.

From: Bringing Heaven Down to Earth by Tzvi Freeman - tzvif@aol.com