In laptop computing, small is beautiful -- until it becomes brutal.

Sure, a three-pound laptop is better to carry than a four-pounder, but it turns out that one or two pounds of portable computer can be too much to bear.

Toshiba's Libretto U100, barely over 2 pounds 3 ounces, and San Francisco start-up OQO's model 01+, at a mere 15 ounces, both represent extraordinary feats of computer design. They pack many functions of a desktop into little more space than a paperback book -- in the OQO's case, an MP3 player. They're guaranteed to draw the attention of neighbors every time you flip them open.

And if only human hands and eyes could be miniaturized to match the diminutive keyboards and screens of these tiny laptops, these things could be as fun to use as to look at.

Of these two, the Libretto -- the latest version of a machine that debuted in 1997 -- seems more promising at first. With a familiar layout of keyboard, pointing device (a touch-sensitive nub like those on IBM/Lenovo ThinkPads) and screen, the tested Libretto U100 ($1,999) could be a regular laptop that shrank in the wash.

Unfortunately, the Libretto's boxy enclosure, as thick as many other laptops (11/4 inches) but far smaller in the other two dimensions (roughly 8 by 6 inches), leaves too little room for its keyboard and screen.

The keyboard defies touch-typing, yet is also too big for the thumb-typing practiced by users of Blackberry and Treo handhelds. Only two-fingered, hunt-and-peck input seems to work.

The screen spans just 71/4 inches across but displays the same resolution as most 12-inch LCD screens, so everything on it looks like it's being viewed through the wrong end of a telescope. Toshiba did not need to put its users through this kind of suffering -- the Libretto's dainty display is surrounded by wide expanses of plastic. A nine-inch display (once a standard size for laptop screens) could easily fit in the Libretto's lid.

The Libretto offers memory, hard drive space and a processor comparable to those of larger machines: 512 megabytes, a 60-gigabyte hard drive (though only 52.7 are available, thanks in part to a hidden recovery partition) and a 1.2-GHz Pentium M chip. A secure digital memory card slot comes built in, but a CD-DVD drive is exiled to a separate, extra-cost module that attaches to the underside of the Libretto and adds another pound.

Beyond a pair of USB ports and one FireWire connector, the Libretto includes WiFi and Bluetooth wireless. But the latter connectivity, used to link such nearby gadgets as cell phones, requires putting up with Toshiba's inept Bluetooth Manager program. (If you try to pair the laptop with a peripheral but forget to turn on its Bluetooth receiver, this application will report that "Bluetooth is not ready" instead of doing something useful, such as, say, turning on Bluetooth.)

Battery life, at 33/4 hours in one test with both WiFi and Bluetooth left on, should have been better. The same goes for the heat put out by this machine: Its left flank and underside got uncomfortably hot in sustained use.

For all its compact contours, the Libretto dwarfs the OQO model 1+, an update to the machine OQO launched last year. It doesn't tip the scales so much as tickle them, weighing little more than its power adapter. "Laptop" isn't even the right word for this machine, 51/4 by 31/2 by 7/8 inches; "kneetop" might be more accurate.

The $1,899 01+ doesn't open like a conventional laptop, either. Its screen occupies the outside of its lid, which slides back to reveal a thumbs-only keyboard.

This 5-inch screen isn't painful to view, thanks to a low resolution of 800 by 480 pixels that keeps text at a readable size. But most applications and Web sites expect a much bigger display, so you wind up constantly scrolling up, down and side to side.

The keyboard feels reasonably comfortable if you're used to a BlackBerry or Treo, but the pointing devices need work. One puts a ThinkPad-style nub to the right of the keyboard with left-click and right-click buttons to the left, requiring a weird two-hand straddle.

After the first day of testing, the nub and buttons mysteriously stopped working, leaving me stuck with the other option, a stylus that slides into the back of the 01+ and that needed a recalibration procedure before it could be used with any great accuracy. (The screen doesn't respond to taps by other objects, such as your finger, so try not to lose the stylus.)

The almost 30 gigabytes of hard drive space and 512 megabytes of memory probably won't get exhausted anytime soon, given the difficulty in using most Windows programs. With no CD or DVD drive -- let alone a memory-card slot -- just installing packaged software would require buying an external drive to plug into the 01+'s USB 2.0 orFireWire port. It also comes equipped with Bluetooth wireless.

The 01+ has no modem, and using a wired Ethernet connection requires plugging in an adapter. That leaves WiFi as the one built-in form of online access -- but the test model's twitchy, on-and-off reception made it all but unusable on my home network.

The biggest disappointment in the 01+ was its miserable battery life. The review unit struggled to last two hours and usually failed to do so, even though its pokey Transmeta processor is supposed to be one of the most efficient chips on the market. And it radiated even more heat than the Libretto.

Given how badly these two machines fail at their assigned roles, should anybody even bother to try to make a computer this small?

Maybe. A device bigger than a handheld organizer but smaller than a laptop -- perhaps running the simpler, smaller Palm or Windows Mobile operating system instead of trying to choke down Windows XP -- could be an effective way to browse the Web and get some work done.

But it can't send its users to their optometrists, its battery can't expire in a couple of hours, and it can't cost as much as two or three regulation-size laptops.

Living with technology, or trying to? E-mail Rob Pegoraro at rob@twp.com.

The new Libretto U100 from Toshiba, above, and OQO's 01+ stuff a lot of computing power into maddeningly small packages, positioning them uncomfortably between laptop and handheld.