The short answer for a young Rex
A young Rex rabbit is sexed by looking at the shape that appears when the vent area is gently pressed: a male produces a round, raised, tube-like opening, and a female produces a slit that opens lengthwise and stays comparatively flat. Coat gives no clue in a Rex — the famous plush, velvety fur feels identical on brothers and sisters — and young kits of both sexes look almost the same size, so this vent check, done calmly and repeated as the kit grows, is the only home method worth trusting.
How to do the check without upsetting the kit
The Rex kit should be cradled on its back in your lap, hindquarters fully supported, never dangled or flipped roughly, because a startled kick from those strong back legs can injure a young rabbit's spine. Work on a low surface over a towel, keep the session under a minute, and use the thumb and forefinger of one hand to press very gently either side of the vent. On a young male Rex the tissue everts into a neat circular cone; on a young female it parts into a narrow slit with a small mound at one end. If nothing clear appears, stop rather than pressing harder — squeezed tissue swells and becomes harder, not easier, to read.
Why young Rex kits are so often mis-sexed
Young Rex kits fool people because the structures are tiny and unfinished: a young male's parts have not yet descended or developed, so he can look convincingly female at four or five weeks, and a plump young female's vent can bulge enough to mimic the male's round shape. Even experienced hands re-check. The reliable pattern is to sex a young Rex once, note the result as provisional, then look again around eight to ten weeks when the difference is unmistakable — and by three to four months a male's testicles usually settle visibly either side of the vent, settling the question for good.
What to do with the answer once you have it
Knowing your Rex's sex changes real decisions: mixed-sex littermates who were harmless nestmates can produce a litter of their own surprisingly early, so confirmed males and females need separate housing plans as they mature, and your vet will plan any neutering conversation around the recorded sex. Write the result down with the date of the confirming check rather than the first guess, and note who confirmed it if a vet or breeder helped. If two checks disagree, treat the later, clearer look — or a professional's — as the one that counts.
When to hand the question to a vet
Hand the sexing of a young Rex to a vet if repeated gentle checks stay ambiguous, if the vent area looks swollen, sore, or injured after handling, or if the kit becomes distressed every time you try. A rabbit-savvy vet settles sex in seconds during the kit's first health visit and can check the vent area is healthy at the same time. Bring your provisional note along so the visit confirms or corrects it on the record. Once confirmed, KinsLe Pet Passport holds your Rex's sex, age notes, and vet details together in one packet you can hand over at any appointment.