\documentclass{book}
\usepackage{hyperref}
\newcounter{testcounter}
\newcounter{experiment}[chapter]
\newcounter{subexperiment}[experiment]

\typeout{\meaning\theHtestcounter}
\typeout{\meaning\theHexperiment}
\typeout{\meaning\theHsubexperiment}

\makeatletter
\providecommand*{\toclevel@experiment}{1}%    diese beiden Zeilen
\providecommand*{\toclevel@subexperiment}{2}% spielen keine Rolle
\def\experiment{%
    \@startsection{experiment}{1}{\z@}%
         {-3.5ex \@plus -1ex \@minus -.2ex}%
         {2.3ex \@plus.2ex}%
         {\normalfont\Large\bfseries}%
}%
\def\subexperiment{%
    \@startsection{subexperiment}{2}{\z@}%
         {-3.5ex \@plus -1ex \@minus -.2ex}%
         {2.3ex \@plus.2ex}%
         {\normalfont\large\bfseries}%
}%
\makeatother

\begin{document}

\experiment{exp1}
\subexperiment{subexp1}


\chapter{titel2}

\experiment{exp2}
\subexperiment{subexp2}

\chapter{Testchapter}

\end{document}