A salt shock of 684 mM NaCl reduced RNA and DNA synthesis to about 30% of the control level in Synechocystis. DNA synthesis recovered to the initial level within 4 h, while for recovery of RNA synthesis about 8 h were necessary. In cells completely adapted to different salt concentrations (from 171 to 1026 mM NaCl), a continuous decrease in the RNA content with increasing salt concentrations up to 684 mM NaCl was found, whereas the lowest DNA content was measured around 342 mM NaCl, i.e., the salinity at which maximal growth occurred. With the uracil and thymidine incorporation technique, maxima in DNA and RNA synthesis were detected in control cells. Comparing these rates with nucleic acid synthesis rates calculated from the contents of DNA and RNA and the growth rates indicated that adaptation to 1026 mM NaCl seemed to lead to an increased RNA turnover in Synechocystis. Analysis of protein synthesis with S-35-methionine labeling showed alterations in salt-adapated cells of Synechocystis. At least three proteins (20.5, 25.8, and 35.8 kDa) were synthesized with highest rates at salinities leading to maximal growth, the synthesis of nine proteins (12.5, 16.9, 19.2, 22.2, 24.7, 28.5, 30.5, 50.3, and 63.5 kDa) increased and that of several other proteins decreased with increasing salinity; but only three proteins (12.5, 22.2, and 30.5 kDa) accumulated under these conditions. The adaptation of Synechocystis to enhanced salt concentrations led also to increased contents of glucosylglycerol, glycogen, and significant amounts of K+ as well as Na+ ions.