Remove space before chapter title - with KOMA script (scrbook)

I am using the KOMA script document class scrbook. Is there any way to make the chapter headings appear exactly at the top of the page body just by using KOMA (without using titlesec, fancyhdr, fncychap, ... for they would mess up other things in my documents)? Is there any way to change the space between section heading and the following text?

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I'm not quite sure what you mean. Are we talking about the chapter titles in the document and that KOMA, like the standard class, adds a vertical space before the chapter or do you refer to the header where KOMa can print the current section/chapter one reads in. In case you mean the position of chapters, search for \chapterheadstartvskip –  Martin H Feb 1 '12 at 16:38

The following is probably what you're after:

\documentclass{scrreprt}
\usepackage{blindtext}% http://ctan.org/pkg/blindtext
\usepackage{showframe}% http://ctan.org/pkg/showframe

Setting \chapterheadstartvskip (default is \vspace*{2.3\baselineskip}) to \vspace*{-\baselineskip} "unskips" the vertical baseline skip to set the chapter title on the first line.
showframe was loaded just to highlight the text block, while blindtext provided dummy text.