Q:

remove boldface from chapter titles, section and subsection names

How is it possible to remove the boldface  from chapter titles, section and subsection names? I have found the following code for sections, but I can't find a way for chapters and subsections.
\makeatletter
 \renewcommand\section{\@startsection {section}{1}{\z@}%
 {-3.5ex \@plus -1ex \@minus -.2ex}%
 {2.3ex \@plus.2ex}%
 {\normalfont\Large}}
 \makeatother

EDIT: Be sure to check the answer I provided in addition to Tobi's and Werner's answers.

A:

Depending on the class you are using there are different more or less easy ways.
If you use* KOMA-Script (which is recommended especially for german texts) you can change the disposition font element:
\documentclass{scrbook}% analogus to book class

\setkomafont{disposition}{\mdseries\rmfamily}

\begin{document}
\chapter{Chapter}
\section{Section}
\subsection{Subsection}
\subsubsection{Subsubsection}
\end{document}

Otherwise the titlesec package may helps
\documentclass{book}

\usepackage[md]{titlesec}

\begin{document}
\chapter{Chapter}
\section{Section}
\subsection{Subsection}
\subsubsection{Subsubsection}
\end{document}

The titlesec package would work with KOMA-Script too …
* A full working minimal working example (MWE) would have showed the class so I don’t have to guess ;-)

A:

You could also patch the standard book document class commands using etoolbox:

\documentclass{book}
\usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
\makeatletter
\patchcmd{\@makechapterhead}{\bfseries}{\relax}{}{}% Non-bold \chapter name
\patchcmd{\@makechapterhead}{\bfseries}{\relax}{}{}% Non-bold \chapter title
\patchcmd{\section}{\bfseries}{\relax}{}{}% Non-bold \section
\patchcmd{\subsection}{\bfseries}{\relax}{}{}% Non-bold \subsection
\makeatother
\begin{document}
\chapter{First chapter} This is a chapter.
\section{First section} This is a section.
\subsection{First subsection} This is a subsection.
\end{document}

The reason for patching \@makechapterhead twice is because the name (Chapter) and title is separately typeset in \bfseries.

