The improved valve assembly controls flow of fluid in "stream sampling" applications. The assembly valve block has at least two inlet passages, an outlet passage and a separate flow control valve for connecting each inlet passage to the outlet passage. Each valve has a single primary valving surface and each of the two inlet passages is selectively placed in flow communication with the outlet passage by positioning one of the primary valving surfaces. Actuation of each valve places the corresponding inlet passage in flow communication with an outlet passage common to all inlet passages. Both ends of the outlet passage are connected together by a "smooth bend" curvilinear tube attached to a fluid analyzer. The assembly can be operated by a control device for "programmed" stream sampling.