SILENT COLLISIONS.

Morphosis has played a constructive role in LA's idiosyncratic architecture scene for two decades now. After a dip in fortunes in the early 1990s (victim of the Japanese recession) and the departure of Michael Rotondi to found RoTo, Morphosis under Thorn Mayne is again to the fore, constructing impressive new projects and the subject of an exhibition at the Netherlands Architecture Institute.

Nothing Morphosis does is especially easy. Having travelled from exhibition last year in Madrid, the drawings and models currently on show in Rotterdam are satellites to a major intervention specifically designed by the firm. This is a vast folded mechanism inhabiting its host - the NAi's Main Hall - as a benign parasite. It moves. Visitors enter onto a bridge leading to a large central platform. Occupied only by an asymmetric chair, this deck is cocooned in splayed white surfaces. Several panels, or wings, rise and fall to a 50-minute time-cycle, slowly changing the nature of the installation from an enclosure to a kind of open viewing instrument. Panels are taut frames of white cloth held by an exposed system of steel cables and motors. When lowered, planes align through low windows to an outdoor fragment sliding into an existing external pool. With nothing but the visitor to inhabit the upper pavilion, Morphosis have celebrated their immediate context with a sample of their own wares.

Beneath this mechanism, down a stately stepped ramp, the firm's drawings and models are displayed in parallel slots of space. The entire undercroft is subdivided by structural partitions set between the hall's deep lateral piers. With projects dating from 1981 to the present, it's quite the etalage down there, a compact retrospective. Viewed now, after many years, there's an almost shocking beauty about the earliest artefacts: the block model of Hermosa Beach with its miniature solids and voids, the meticulous elevations of the Lawrence Residence that reveal (again) one building nestling inside another. All embody that quintessential Morphosis conjecture of ground and urban form, of descriptive geometry and appropriated or invented objects.

Mayne and his colleagues constantly integrate things into richly stratified environments. From the early '90s onwards, they begin to combine walls and roofs into curvaceous envelopes; the Yuzen Car Museum, for instance (AR September 1992), and urban propositions for Paris and Vienna. Then Morphosis embraced the computer and the practice now have the means to envisage new constructs, unimaginable in the days of drawings boards and orthodox model-making. Two recent buildings, the Diamond Ranch High School east of LA and the Hypobank mixed-use development in Klagenfurt, Austria, are fusions between topography and cybernetics, in which it's deliberately unclear where ground ends and building begins.

Projectors within the installation at the NAi are intended to show clips of newly evolving projects. The entire contraption ('1500 metres of aluminium sections, 600 square metres of cotton fabric, 1000 square metres of MDF and more than 10 000 screws and bolts') may itself be anachronistic and indeed was not fully operational during initial days of the exhibition. Delicate and presumably computer-generated, it resembles those fossil-like apparatuses in Mayne's work of a decade or more ago. Yet rather than blurring boundaries between architecture and site, it allows the visitor a look at the NAi Main Hall anew. It's greatly to Morphosis' credit that Jo Coenen's building has never seemed so interesting.

Morphosis: Work-in-Progress, Netherlands Architecture Institute (NAi), Rotterdam, 4 September 1999 to 16 January 2000. The exhibition is accompanied by a new monograph on the practice published by Rizzoli.