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We present results of a continuing study of milliarcsecond structure of extremely high-redshift sources aimed to investigate possible cosmological evolutionary effects of quasars and to use the milliarcsecond structure of extragalactic sources in cosmological tests. Four quasars at redshifts greater than 3: 0336$-$017 ($z = 3.197$), 0636+680 ($z = 3.174$), 1442+101 ($z = 3.531$), and 2048+312 ($z = 3.185$) were observed with a global VLBI network at 6 cm. All four sources are resolved, two sources, 0636+680 and 1442+101, show well defined jets, while two others, 0336$-$017 and 2048+312, demonstrate some indications on possible jets.
The National Astronomy and Ionosphere Center is operated by Cornell University under a Cooperative Agreement with the National Science Foundation. The Netherlands Foundation for Research in Astronomy is supported by the Netherlands Foundation for Scientific Research (NWO). The National Radio Astronomy Observatory is operated by Associated Universities, Inc. under a Cooperative Agreement with the National Science Foundation.
